I often read on this forum about 'bumping back' the shoulder. I have never done it but have only loaded for one 'modern' cartridge - the 223 Remington. Why should the shoulder be 'bumped back'? Is this only applicable to high pressure cartridges or do some rifles lack the camming on closing required to chamber a 'neat' fitting case?

My Lee Enfield, if chambering a case originally fired in a different rifle, then full length resized, will chamber tightly but will loosen up on firing.

I do it one multiple calibers with a Redding body die...

It bumps the shoulder back without touching the case neck...

you do so when the cartridge chambers tightly or won't chamber at all..

by using a body die and then a Lee Collet Neck sizer...not only is the brass more uniform that when using a Full Length Die...

but it also increases case life dramatically over a F/L die, as it works the brass much less..

a body die also prevents the need to trim the brass frequently...

quote:

you do so when the cartridge chambers tightly or won't chamber at all..

So it's not a routine thing then. But why would a case become 'unchamberable' or tight if fired in the same rifle?

With higher pressure loads, brass flow is normal, and the shoulder naturally moves forward. After three or five neck-sizings, the shoulder has to be pushed back. You may not be shooting at high enough pressure to cause a situation. Actually, if you're FL sizing, you're probably bumping each time.

I'll resist the temptation to kid you about self-adjusting locking lugs. Won't even say anything about the equator being involved. It is a temptation, though. Haven't had an equatorial humor fix lately.

Shoot and reload for a Swiss K31 and you'll answer your question. Not all rifles/cases require it, but you won't rechamber the 7.5, if the case isn't resized fully including the "bumping back" of the shoulder (at least with my copy).

I can usually get the proper case dimemsions by full-length resizing about a half-turn beyond shellholder contact.

In addition to case expansion from pressure some rifle actions flex a little during fireing and allow the shoulder to move. Levers are one of the more common types that can do this. The expander ball also in some instances pulls shoulders forward and a little bump helps keep them in proper postition.
I agree it is not something that is always needed and I too have loaded a lot of different cartridges quite a few Xs with no chambering trouble arising from not bumping the shoulder (collet dies in 708, 06, 223,6.5). It is needed at times though, and one should be aware of the fact.

Bumping the shoulder is generally associated with neck sizing. After a few neck sizings, the case begins to chamber harder. A body die will then bump (push back) the shoulder slightly and squeeze back the case body, without touching the neck.

The other instance is when guys fl size but don't have the die turned down enough to bump the shoulder. I usually find it takes 2 firings of a virgin case with a strong load for the case to stretch out to the dimension of the chamber. Once it's there and you fl size you will squeaze the shoulder forward slightly as the case resizes in the die.
This is when everyone gets on the forum and says "how come my fl brass wont' chamber"?
AND the answer that usually fixes the problem is when someone says turn your die down slightly.
If you like reloading I HIGHLY RECOMEND you get a headspace measuring tool like that from hornady for about $35 or from Larry Willis so you can see how your brass grows with ea firing and resizing. I believe there is a slight accuracy gain by just bumping the shoulder about .001-.002 so that it chambers easily yet "fills the chamber" better as it waits to be fired.
I use lee collet dies and redding body dies set to do this in my fav calibers and love the results.

All brass flows above roughly 40kPSI and all actions flex or stretch depending on the pressure, some more than others, which means the brass will stretch also and soon be too long for a chamber at rest.

Many, if not most, factory dies set up the way the instructions say WILL push back the shoulders, the amount depending on many factors...so shoulder "bump" is system inherent.

Some brands of neck dies DO set the shoulder back and some don't, and you can adjust the amount of shoulder "bump" by either grinding a few thou off the bottom of the die or by using Redding Comp Shell Holders.

For the average hunter it is a non-issue except to be sure the round will chamber.

The farther you get into competition and "bench prepping" your brass, the more you will get into "MINIMAL' case crunching in all directions by fitting the die dimensions to the chamber dimensions.

To get the maximum accuracy out of a rifle SYSTEM you need to "tune" every aspect of the system...scope, rifle/stock, ammo etc...

Bumping back the shoulder 0.001" - 0.002" makes the ammo more uniformity and gives it longer life.

Whether or not you get cross-eyed and anal retentive doing all the minutia required to produce a bugholer depends on your level of accuracy requirements and you ability to muck about with mind bending consentration to produce, at least, the ammo part.

I used to do the process with my benchresters and still to do most of the process with my varminters, but just make sure the rounds feed through the action without any problems with my hunting rifles from 6mm up.

Many times there is more emphasis placed on something that sounds cool and is only marginally effective, while totally forgetting about all those factors that do play a leading part, but are mundane or un-cool.

Ammo closely sized to the individual chamber is said to give the best accuracy. Over time, however, the case will become so closely match the chamber that it will be hard to chamber. Bumping the shoulders back was originally associated with hunting ammo. The one paramount thing you must have with hunting ammo is absolute reliabilityh in feed and function. The bench crowd could take the time and effort to cam a tightly fitted cartridge into battery. Not so the hunter. His ammunition must slide effortlessly into the chamber for the fleeting opportunity at a trophy or a rapid shot to avoid dismemberment.
Moving the shoulders back slightly is supposed to give the best of both worlds. Brass that is closely fitted to your rifle yet will chamber readidly.
That's it only purpose.

As an illustration...way back in the mists of time when I was young, dumb and didn't shave yet, I acquired a 308 cal Rem 788...I also bought a set of Lee "hammer" dies. The rifle would do MOA even loading ammo with those very primitive but very usefull dies. After about 3 reloads of "normal" loads or 1 with a "magnumitis", load the bolt wouldn't close on an empty case once it was ejected.

I took my whine and my rifle to my favorite 'smith who was wise in the ways of dumass youth.

Much to my surprise he didn't laugh or look at me like I was pond scum...he very seriously gave me a song and dance which I didn't understand hardly and also an old Lyman 308 sizing die and told me to use it every 3rd firing and NOT to do "magnumitis" again with that particular rifle.

I used one load and one bullet for 25 years to take anything that moved and ALWAYS used that Lyman sizer to size the case after the 2, 5, 8 and sometimes 11th firing of any case that lasted that long.

The same 'smith also inducted me into the "secret world" of BENCHRESTING and the need to mic and neck turn cases after a few firings.

I soon learned this sport is worse than drugs and will keep you poor the rest of your life.

Have somebody walk you through the brass bumping. Too much bumping can cause case separation. I have had brass that was too fat at the base and felt like it needed bumping. I have a die that I made to size the base smaller when needed.
Butch

Thanks for all the informative and interesting posts! I guess I must be just lucky to have a quality design rifle that doesn't need case bumping! (And it's probably one of the most flex-actions).

It's interesting that my mini-14 never required shoulder bumping.
Come to think of it, I never needed to trim cases either - for any of my guns!? (I didn't load THAT mildly! My loads were pretty reasonable performers - still are).

quote:

Originally posted by 303Guy:
Thanks for all the informative and interesting posts! I guess I must be just lucky to have a quality design rifle that doesn't need case bumping! (And it's probably one of the most flex-actions).

It's interesting that my mini-14 never required shoulder bumping.
Come to think of it, I never needed to trim cases either - for any of my guns!? (I didn't load THAT mildly! My loads were pretty reasonable performers - still are).

303Guy-

Your problem seems apparent.... You are using good working loads, not trying to make a death ray out of your rifle. Maybe you don't understand the currently popular school of thought which holds that all factory cartridges and published loading data are grossly underpowered, and need to try jacking your loads up into the warp speed category?

I don't believe your experience is based on your "Smelly" action. Rather, I think it is a sign of a good all-around serviceable load. A little anecdote:

In 1959, I bought 1,000 mixed headstamp, fired, G.I. .30-06 brass. I sorted it by headstamp, tumble-cleaned it, and removed all the primer-pocket crimps. Then I used some for '06 reloads, and the rest to make cases for .244 Remington, .257 Roberts, .25-06, .270, 7x57, 7x64, 8x57 (both J and S), 8mm/06, 9x57, and probably a few other rifles I have forgotten.

Guess what? I have NEVER had ONE of those rounds that didn't want to chamber after firing, no matter how many reloads. I was taught early to always use a full-length size die, but to never adjust it down to the point that it bumped the shoulder back.

That is easy to determine if you use an "oil consistency" sizing lube when one is first sizing the cases...the depth the die is sizing the neck is seen as a faint black line of lube all the way around the neck of the case. I adjust my dies to where that line is just clearly above the top corner of the shoulder/neck junction.

And, since I've had no trouble with the cases chambering, I've never had to bump the shoulders back in any of that brass....and this is the 50th year it has been in use....not shot every year in every version, but still shot from time to time in all of them. Note, these are all for hunting rifles, not for BR rifles.

I definitely never bump back the shoulders in my BR brass, because I don't even own either neck size or full-length size dies for my BR rifles. I do own case forming dies for the BR guns, and do a very careful job of making my cases for them,but once they are made, the cases are never sized again at all, let alone enough to bump the shoulders back. And some of my BR cases have been fired over 1,000 times each. (I know that sounds unbleieveable, but it IS true.) And my favorite load is a warm one in them....moving 215 grain .30 bullets at 2309 fps chrono average from the little 1.5" long .30-BR cases.


Anyhow, I am not a believer in operating rifles at balls-to-the-wall maximum pressures. I actually prefer loads which are just enough over the beginning listed loads to produce good consistent hunting accuracy. I still believe that moderate loads have a lot of advantages...easier on the guns, easier on the brass, still kill well, less recoil, less constant putzing with the ammo components, and on and on. If I want/need more power, I use a bigger bullet and cartridge case.

Sounds like you might come from the same school yard as I.

quote:

Originally posted by kraky:
The other instance is when guys fl size but don't have the die turned down enough to bump the shoulder. I usually find it takes 2 firings of a virgin case with a strong load for the case to stretch out to the dimension of the chamber. Once it's there and you fl size you will squeaze the shoulder forward slightly as the case resizes in the die.
This is when everyone gets on the forum and says "how come my fl brass wont' chamber"?
AND the answer that usually fixes the problem is when someone says turn your die down slightly.
If you like reloading I HIGHLY RECOMEND you get a headspace measuring tool like that from hornady for about $35 or from Larry Willis so you can see how your brass grows with ea firing and resizing. I believe there is a slight accuracy gain by just bumping the shoulder about .001-.002 so that it chambers easily yet "fills the chamber" better as it waits to be fired.
I use lee collet dies and redding body dies set to do this in my fav calibers and love the results.

+2

except for the Larry Willis thingy.

JUST KIDDING kraky! I'm sure the Larry Willis headspace thingy works just fine, do you have one?

No I don't have the Larry Willis thingy but the "cool factor" vrs playing with bushings etc is probably on par with the price diff.....I've been a proponent of his magnum collet die which has saved my friend a ton of wby brass...maybe I can swing a deal with Larry for a demo model!!!!!!!!

Have we determined that you're not PFL sizing? Do you own a neck-sizing die? Is your press mounted upside down to neutralize the equatorial influence?

quote:

Originally posted by 303Guy:
...Why should the shoulder be 'bumped back'? Is this only applicable to high pressure cartridges or do some rifles lack the camming on closing required to chamber a 'neat' fitting case? ...

Hey 303Guy, As you can see, people have all kinds of ideas on why and how to go about doing it. Since you load completely different than I do, I'm not sure what I think is normal will mean a lot to your methods.

As for the "Why?", it appears to me it is because most of the people who "Bump Back the shoulder" are Neck Sizing(NS). As the number of shots on a specific Case increases, it still Expands due to the Pressure, but Contracts a bit less each time. So, when they NS, they are only retightening the Neck and not returning the Shoulder to a slightly shorter dimension. Over time, this causes the Bolt to close with a bit more difficulty on a "typical" Case.

As usual, there are exceptions to the rule, and some Chamber/Die combinations allow constant NSing and never having to Bump Back. Alberta Canuck mentioned this the other day in a different thread.

I would also think the Intensity level of the actual Load would affect the second part of your question, but again there seems to be exceptions.

For sure, the people that Bump Back every X-number of shots have a constantly changing Case when they shoot. Lets say they Bump Back after every 5-shots. Then shot 1 is different from shot 2 because the Case is changing dimensions. Not much, but it is changing. Same between each shot on the same Case until the sequence begins again.

Since I P-FLR for all my Bolt Actions, this is automatically compensated for simply due to the FL Die adjustment. So, I suppose a person could think of P-FLRing as "slightly" Bumping Back with each resizing. I don't think of it like that because it would be in conflict with "why" most people use the term Bumping Back to start with.

Best of luck to you.

On my 6ppc I use a die that I made myself. It sizes the diameter of the shoulder .002, the base at .003 and that is with a .001 bump every time. I had Dave Kiff of Pacific Tool grind a sizing die reamer that would do that based on my chamber reamer that he also ground. This way I do not have to let my brass grow a lot before sizing and am not overworking the brass. We normally shoot at around 72,000psi.
Butch

And then there are the BR shooters who may or may not bump back but do throw away their brass and make new, tediously perfect, brass after every match. (I suspect in the Obama world that brass availability is reducing their numbers, though.)

Thank you all.

Some folks seem to think that we here on the opposite side of the equator are 'upside-down'. No not at all. This is how I mount my press. Take a look!


↓ --------------- Equator side --------------- ↓

Ifn I were to mount it the other way, the powder would fall out the case!

You see, it's like this. Is this a six or an upside-down nine? 6 or 9

quote:

And some of my BR cases have been fired over 1,000 times each. (I know that sounds unbleieveable, but it IS true.) And my favorite load is a warm one in them....moving 215 grain .30 bullets at 2309 fps chrono average from the little 1.5" long .30-BR cases.

It is believable but astounding just the same!
Ummm... was that a typo or do my eyes deceive me - 215gr, 2309fps, 30BR (308 x 1½)? Holy Cow! How is that possible? That would make it a rather efficient cartridge and load. Interesting!

It seems there is some missing link in our (well, mine anyway) understanding on what actually happens when a cartridge is fired. Some cases need to be 'bumped back' some not. Some cartridges produce a power quite unexpected. Mmmm.....
Then we have the fellow who rechambers a 270 to 30-06 and shoots factory ammo with good accuracy. (Or something like that). 30-06 on a 270 barrel! Maybe us mere mortals cannot comprehend the massive pressures and forces involved!

On the "other side" of the equator I'm known as Winchester 96.

For what it's worth, there's lots oh highpower match rifle shooters who full length size their fired cases fired in virtual SAAMI chambers who bump shoulders back a couple thousandths all the time. They reduce body diameters on their rimless bottleneck cases a couple thousandths in dies whose neck diameter's a couple thousandths smaller than a loaded round has.

Tests of this ammo with rifles clamped in free recoiling machine rests (shoulder fired match rifles aren't made to be shot free recoil off rests atop a bench) have shot 10 to 20 shot groups smaller than long range benchrest records. And case life is several dozen loads. I've got as many as 45 but ran out of powder at the test site. Some folks have got over 70 with .308 Win. cases.

Sierra Bullets' has been reloading their rimless bottleneck cases used to test their products since the early 1950's this way. They used to lap out the necks of full length sizing dies but now use Redding "S" Full Bushing dies for calibers that have them. Otherwise, they use standard full length sizing dies. All their bullets are tested with rail guns whose barrels have standard SAAMI chambers. I've seen some of their 30 caliber HPMK bullets' test groups in the low ones at 100 yards from .308 cases.

303 Guy ponders.....

quote:

It seems there is some missing link in our (well, mine anyway) understanding on what actually happens when a cartridge is fired.

Well, hear goes my try at explaining.

I'll start with how the round's positioned in the chamber when the bolt's closed.

Closing the bolt on a rmiless bottleneck case causes a few thing to happen. First off, the back of the case is pushed against the chamber wall by the extractor; up for some, to the side for others with Mauser style claw extractors. The rim may stop at the edge of a recessed bolt face, too. It's a bit hard to tell without being able to measure exactly where the back of the case is. Second, if the bolt's got a straight line plunger style ejector, that pushes the round as far forward as possible. There's typically enough clearance in the extractor's lip to let the case shoulder be pushed into and stop against the chamber shoulder which centeres the front of the round quite well in the barrel. Third, when the firing pin's released, it smacks the primer pretty hard; hard enough to drive the case forward setting the shoulder back a few thousandths; how much depends on firing pin spring strength, pin weight, shoulder angle and how smooth and slippery the shoulder mating surfaces are. It's usually in the range of 4 to 8 thousandths. This situation is why the Brits couldn't use a rimless version of the 30 caliber rimmed double rifle cartridge for bolt guns to load it reliably; they put a belt on it and called it the .300 H&H Magnum.

Now the round starts its firing sequence of events.

First, the primer detonates (burns very, very fast like a smacked match head and ignites the powder which builds up pressure inside the case. The first part of the case to expand is the front part where brass is the thinnist. Second, the shoulder and front part of the body soon press very hard against the chamber wall an at the same time the back of the case starts to stretch back. There's enough pressure in the case to push the primer out a few thousandths at this time. Meanwhile, the neck expands against ehe chamber as the bullet's pushed out by gas pressure and goes on down the barrel. Third, as more of the case body presses hard against the chamber wall, the case head eventually stops against the bolt face. Pressure's highest when this happens and this is what causes most of the stretching about 2/10ths of an inch up from the head; incipient head separation starts (and will continue to get worse if the case shoulder's set back too far when full length sizing it). If peak pressure's not enough to push the case head against the bolt face, the fired case will have its primer protruding a little bit; as much as several thousandths with a powder charge about 15% less than maximum. Fourth, pressure drops, then the bullet exits the muzzle and the case springs back a bit from the chamber walls; how much depends on how brittle the brass is and how far it was alowed to expand.

quote:

Originally posted by Bart B.:
303 Guy ponders.....

quote:

It seems there is some missing link in our (well, mine anyway) understanding on what actually happens when a cartridge is fired.

Well, hear goes my try at explaining.

I'll start with how the round's positioned in the chamber when the bolt's closed.

Closing the bolt on a rmiless bottleneck case causes a few thing to happen. First off, the back of the case is pushed against the chamber wall by the extractor; up for some, to the side for others with Mauser style claw extractors.
Not if the case is neck sized or Partial Full Length Sized. With neck sizing, the case body has not been sized and is very close to chamber size and will resist being pushed to one side by the extractor. In PFLR, the contact at the shoulder between the case shoulder and chamber shoulder will hold the case head back against the bolt face and the conical shape of the shoulder will keep the case from being pushed by the extractor.

The rim may stop at the edge of a recessed bolt face, too. It's a bit hard to tell without being able to measure exactly where the back of the case is. Second, if the bolt's got a straight line plunger style ejector, that pushes the round as far forward as possible.
Again not with PFLR as the case can not move forward if held in stasis between the shoulder and case head.

There's typically enough clearance in the extractor's lip to let the case shoulder be pushed into and stop against the chamber shoulder which centeres the front of the round quite well in the barrel. Third, when the firing pin's released, it smacks the primer pretty hard; hard enough to drive the case forward setting the shoulder back a few thousandths; how much depends on firing pin spring strength, pin weight, shoulder angle and how smooth and slippery the shoulder mating surfaces are. It's usually in the range of 4 to 8 thousandths.
I have never heard of this and on the face of it find it highly unlikely that the firing pin could push the shoulder brass back .004" to .008"! I need to raise a flag on this one Where is your data or proof that this is happening?

This situation is why the Brits couldn't use a rimless version of the 30 caliber rimmed double rifle cartridge for bolt guns to load it reliably; they put a belt on it and called it the .300 H&H Magnum.

Now the round starts its firing sequence of events.

First, the primer detonates (burns very, very fast like a smacked match head and ignites the powder which builds up pressure inside the case. The first part of the case to expand is the front part where brass is the thinnist. Second, the shoulder and front part of the body soon press very hard against the chamber wall an at the same time the back of the case starts to stretch back. There's enough pressure in the case to push the primer out a few thousandths at this time. Meanwhile, the neck expands against ehe chamber as the bullet's pushed out by gas pressure and goes on down the barrel. Third, as more of the case body presses hard against the chamber wall, the case head eventually stops against the bolt face. Pressure's highest when this happens and this is what causes most of the stretching about 2/10ths of an inch up from the head; incipient head separation starts (and will continue to get worse if the case shoulder's set back too far when full length sizing it). If peak pressure's not enough to push the case head against the bolt face, the fired case will have its primer protruding a little bit; as much as several thousandths with a powder charge about 15% less than maximum.
All very good reasons for PFLR or Neck Sizing (if it all is true, sounds plausible, but unverifiable)

Fourth, pressure drops, then the bullet exits the muzzle and the case springs back a bit from the chamber walls; how much depends on how brittle the brass is and how far it was alowed to expand.

Hardly any of this is applicable for a belted case either. Most of it is plausible for a new case or a Full Length Resized case.

quote:

Originally posted by 303Guy:

quote:

And some of my BR cases have been fired over 1,000 times each. (I know that sounds unbleieveable, but it IS true.) And my favorite load is a warm one in them....moving 215 grain .30 bullets at 2309 fps chrono average from the little 1.5" long .30-BR cases.

It is believable but astounding just the same!
Ummm... was that a typo or do my eyes deceive me - 215gr, 2309fps, 30BR (308 x 1½)? Holy Cow! How is that possible? That would make it a rather efficient cartridge and load. Interesting!

It seems there is some missing link in our (well, mine anyway) understanding on what actually happens when a cartridge is fired. Some cases need to be 'bumped back' some not. Some cartridges produce a power quite unexpected. Mmmm.....
Then we have the fellow who rechambers a 270 to 30-06 and shoots factory ammo with good accuracy. (Or something like that). 30-06 on a 270 barrel! Maybe us mere mortals cannot comprehend the massive pressures and forces involved!

I don't know why that velocity is possible, but it is. Part of it is doubtless because cast bullets are much easier to accelerate than jacketed bullets of the same weight...probably less bore/bullet friction and less engraving force required both playing a large role.

Second may be the way I use my cases...no neck tension at all, no resizing of the cases so among other things I get full advantage of their capacity. Another part may be that I use the late Don Eagan's MX4-30ARD bullets which are a very long bore-riding-nose bullet with only a small length at the base being of groove diameter. I cast them of pure linotype and sort them by weight after lubing, sizing and gas-checking into 1/10th grain lots.

I also have full obturation of the bore before the round is fired, as my rounds are a very slight interference-fit in the chanber and throat, so there is no gas leakage past the bullet or the case at any point in the firing sequence.

Also, as my cases are touching at all points in the chamber before firing (a friction fit), they therefore do not spring back after firing...they have no where to spring to. Chambering them is much like pushing the loaded round into a FL sizing die without a decapping pin (but not as much effort). When fired, it is as if they are being fired inside a FL sizing die.

Lastly is probably the powder used...VihtaVouri N-135 seems to be a perfect propellant for heavy bullets in that particular case.

Some of my friends using the same bullet in chambers cut with my reamer get different velocities, but not greatly so. One of them uses 2230-S for a powder and shoots his 215 gr. bullets at about 1,950 fps...another uses N-135 just as I do, but shoots his bullets at about 2,150 fps. They both can get higher velocities (and have done so), but find their best accuracy in THEIR rifles at the speeds they now prefer.

Oh, almost forgot...my match rifles have 29.25" long barrels, which barely make it inside the 14 lb. rifle weight rule for CBA "heavy" class. That has to help a lot too.

So, what I have been describing are not practical "field" rifles, but they are darned practical (and winning) cast bullet benchrest rifles.

I also feel that the type of chamber and cartridge match I describe here is the reason my brass lasts so long. It doesn't get work hardened by constant expansion and then resizing. Essentially there is none of either.

It can be damned dangerous, though, if a person isn't used to working to .0001" or smaller dimensions.

Edited to add: Forgot to mention that I chamber & fit my own barrels. First I push the reamer in 'til I can just close the bolt on a ready to shoot case but with no bullet, primer, or powder.

Then I put a bullet in the case with the base about .030" above the neck/shoulder juncture. Then I push a .311" throating reamer into the .299"/.300" Hart bore until I can just close the bolt on a dummy "loaded" round (bullet and case, but still no powder or primer). The cases are neck turned to hold the .311" sized bullet bases with the case neck outer walls just a friction fit with the chamber neck's inner walls. So, basically the bullet nose is just touching the rifling, and the diameter is touching all the way around. Then, the instant the bullet starts to move it is fully obturating the bore.

It's a lot of close work, but also a lot of fun to see those bullets all proceed into the same hole on the paper. And I find the barrel chambering/fitting and case-making a lot of fun too.

quote:

I have never heard of this and on the face of it find it highly unlikely that the firing pin could push the shoulder brass back .004" to .008"! I need to raise a flag on this one Where is your data or proof that this is happening?

Any reasonably smart reloader with the right tools could figure out how to prove this happens. Have at it. Then you'll have your very on verified data and not have to rely on mine which you would probably challenge anyway. Now pull down your flag and get on with intelligent testing and experimenting.

quote:

If peak pressure's not enough to push the case head against the bolt face, the fired case will have its primer protruding a little bit; as much as several thousandths with a powder charge about 15% less than maximum.

quote:

All very good reasons for PFLR or Neck Sizing (if it all is true, sounds plausible, but unverifiable)

I'll agree that it's unverifible by folks who don't know how to cut a powder charge by 15% (or thereabouts) then load it and shoot it in their rifle and measure how much the primer sticks out.

quote:

Originally posted by Bart B.:

quote:

I have never heard of this and on the face of it find it highly unlikely that the firing pin could push the shoulder brass back .004" to .008"! I need to raise a flag on this one Where is your data or proof that this is happening?

Any reasonably smart reloader with the right tools could figure out how to prove this happens. Have at it. Then you'll have your very on verified data and not have to rely on mine which you would probably challenge anyway. Now pull down your flag and get on with intelligent testing and experimenting.

quote:

If peak pressure's not enough to push the case head against the bolt face, the fired case will have its primer protruding a little bit; as much as several thousandths with a powder charge about 15% less than maximum.

quote:

All very good reasons for PFLR or Neck Sizing (if it all is true, sounds plausible, but unverifiable)

I'll agree that it's unverifible by folks who don't know how to cut a powder charge by 15% (or thereabouts) then load it and shoot it in their rifle and measure how much the primer sticks out.

Well, being a reasonably smart reloader and having a few tools (thingy's to HC) laying around, I took a 4 times fired 280AI case with the spent primer still in it


and measured it with a Hornady Headspace Gauge


then chambered and pulled the trigger which caused the firing pin to impact the spent primer. Probably with the same amount of impact as it would have been had it been a live primer


and remeasured it with the thingy


OOPS! Must have done something wrong cause the headspace was exactly the same! So I did it again and damn if it wasn't exactly the same again.

Now it looks like even though the firing pin was hitting close to the same hole that if there was normally enough force to push the shoulder back .004" to .008" then I would have gotten at least a movement of .001" or so, don't ya think? Or maybe you don't.

I can't question the primer backing out with a 15% undercharge (since I have never done that and don't plan to) and that may be one of the things that is correct in your rambling self gratifying dissertation. But since I have repeatedly witnessed your penchant for inaccuracies (should I post links?), I will hold judgement on that for now.

Wow! Thanks for that, Alberta Canuck. I think I undersand how that small case does it. Slow powder, small enough capacity to get enough pressure to burn that powder and a long enough barrel to utilize the energy released!

I do a similar thing with my .... ummm.... somewhat unusual hornet loading method. Only I don't have a 'tight fitting' chamber. Instead, I take up the clearance between the case neck and bullet with a 'waxy-lube' soaked paper cup. (It's not a bench rest rifle, has a rust damaged bore and sure as hell does not get bench rest accuracy! I am planning on trying cast bullets in it - of my own design).

Woods

My fired primers look like those in your pics. That's my normal operating pressure.

About the firing pin pushing the 300 H&H case forward - Bart B. is correct. That case does not have enough shoulder angle to hold the case during firing pin impact and given the clearances required it would be very difficult to provide reliable headspacing on an African game rifle - some of it dangerous. All materials have some degree of elastic compressibility. Using the tan rule and the 12.5º shoulder angle, the forward movement of the case would be 4½ times more than the compression of the brass shoulder. But far worse, the radial clearance produces a longitudinal clearance 4½ times greater. I would think though, that the shock effect of the firing pin strike would be largely absorbed by the indentation of the primer so the compression forces would not produce an appreciable forward movement but will be real! (But the 4 - 8 thousandths of which Bart B. speeks is the difference between a case loosely pressed or held in the chamber and being thumped up hard by the firing pin force. That is a 'springyness' a lot less than the Moulus of brass would suggest as it stems from small points of contact. I'm not sure how much of that is accounted for in headspace clearance. That is, does it add to the headspace clearance? .004 - .008 is a lot more than I would expect unless that applies specifically to the 300 H&H shoulder. (Mmmm .... think of the amount a bolt gets tightened after the clearance has been taken up! And a case is much thinner and is hollow! Mmmm .... !

I hope all that rambling serves to clarify rather than to further confuse! (Hell, I just confused myself! )

I have heard it said that the reason for the H&H belt was to facilitate magazine feeding of what otherwise would have been a rimmed cartridge. That doesn't make sense as all they would have needed to do would be to go rimless!

PS, thanks for the interesting information, all.

Bart B. I still don't understand what really happens inside the chamber when a cartridge is fired. (I'm refering to the rocket science that goes on! - Way beyond such mere mortals as myself. But, I never considered the compressibily of the case on being struck by the primer! Although I did realise the value of preloading the chambered round .... mmmm ...? So thanks for pointing that out).

Hey 303, Bart B did not say that the case was moved forward .004" to .008", he said that the "shoulder was set back" on the case that much, like setting the shoulder back when sizing.

The flag's still up

Ah. OK, but maybe that is what he was refering to. Still, he's got me thinking ... !

Woods -

The phenomenon of shoulder set-back which
BartB. describes in his posts has long been well-known to occur with reduced loads, though it may not be caused by the strike of the firing pin.

It was described early on by the NRA technical staff in the early years of The Dope Bag section of the American Rifleman, back when their regular contributors & advisors to that section were people who had managed major military armouries and ran internal ballistics labs. It has also been described very many other places up through at least the early 1960s by other very credible authorities in the firearms industry. It may or may not also have been mentioned in the NRA
published "Handloader's Guide", but I haven't the time to go re-read it right now to find out.


(No, I am not going to go everywhere possible to find references to the problem. But if you do some research, you WILL stumble across them from time to time....of course they will be tough to find when they are what you actually want to see...life is like that, dang it. When you don't need the info, you'll stumble over it everywhere, it seems.)

Anyway, I suspect the mechanism of the set-back, and the reason you couldn't repeat it with your test, is that it is likely caused by the energy released by the live primer, not the firing pin and its spring.

When the primer is fired, it releases a surprising amount of energy...enough to drive a jacketed bullet completely into a bore on some occasions. The same energy drives the primer back out of the case, and THE CASE FORWARD into the chamber (and somewhat off of the primer). It is the old "equal and opposite reaction" trick learned in high school physics.

In full loads, when full burn pressures are reached and maintained for an instant, the case is then driven back against the bolt face and back over the primer, while the shoulder of the case is driven forward against/into the shoulder of the chamber and held there while the rest of the case moves back.

So, in full loads the case actually shortens then lengthens, but with reduced loads, it may just shorten.

That is perhaps related to why it sometimes takes several firings to fully fire-form some wildcat shoulders too, if too small a charge is used. There just isn't enough pressure with weak loads to force the case to expand in all directions to completely fill the chamber.

It is also why many handloading sources tell beginners NOT to use cases originally used for "squib" loads for full loads at a later date. Though the cases may have had correct headspace when new, their case length to the shoulder may not be adcequate for safe full loads after they have been fired with weak ones.

So to over-simplify it....firing the primer does produce enough "instant energy" to drive primer and case in opposite directions, in some instances probably causing the shoulder set-back which has long been noted. Then, when the powder does its burn, if there is too little powder, there is not enough pressure apparently to blow the case outward enough to completely fill the chamber.

Probably the only way to really test that out is to measure a rimless round's headspace with something like an RCBS adjustable cartridge headspace gauge, then fire the same round with a very reduced charge, but a good strong primer (like a Federal 215 if you can find any these days). Then re-measure the cartridge case's headspace, after firing, with the RCBS adjustable gauge.

I would try it with something like a .30-06, as that is where I have most often seen it reported as happening.

Note, I m not trying to support either side in this "discussion" (argument?). Just trying to confirm that the phenomenon has been long reported, and suggest one possible explanation.

Of course there may be many different reasons and ways the underpowered loads end up SOMETIMES displaying shoulder set-back. If so, then perhaps loading hotter MAY not be the only way to avoid the situation. But loading hotter does seem long accepted by the industry as a workable answer.

Best wishes to you both.

Hey AC, thanks for that explanation.

Bart B did not say that the exploding gases in the primer had anything to do with it, rather that the firing pin strking the primer did

quote:

Third, when the firing pin's released, it smacks the primer pretty hard; hard enough to drive the case forward setting the shoulder back a few thousandths; how much depends on firing pin spring strength, pin weight, shoulder angle and how smooth and slippery the shoulder mating surfaces are. It's usually in the range of 4 to 8 thousandths.

that is why I raised the flag. I'm the first to admit that I might have to lower it later but IMO there is no way a firing pin strike (not the primer exploding) could have enough force to set the shoulder back.

So the primer gases ganged up in the primer pocket trying to get out the primer hole push the case forward and that sets the shoulder back and if there is not enough of a charge to fully expand the case back to the bolt face then the headspace can shorten. Hmmmmm.....

Woods, BartB, and others here -

Thought I'd do a little browsing, so I looked around and found this: (Note, Ed Harris both worked for the American Rifleman as technical staff, but was also an industry insider as well.)

---------

Here's Ed Harris (former American Rifleman staffer) on reduced loads in rimless cases:

"Rimmed cases avoid the common problem experienced when using rimless cases for light loads, in which primer blast drives the case forward, setting the shoulder back and shortening the body length. Rimless cases used repeatedly for reduced loads eventually experience excessive headspace which leads to misfires or case head separations, unless flash holes are enlarged. However, if you mix these cases up and assemble a full load in them it may cause an accident, so using an unmodified, rimmed case for light loads makes more sense."

----------

BartB was not being "dumb" to say the shortening was caused by the firing pin striking the case. He probably read articles where several "experts" said it. That has been said in print many times over the years and I have frequently read it myself. It just dosn't happen to be the most common cause, if firing pin strike is truly a cause at all.

It is also worth noting that this phenomenon does NOT occur with either belted or rimmed brass, which is one of the advantages of belted brass. They don't headspace on the shoulder until already fired, and the rim or belt keeps them from being driven forward enough by the primer explosion to do any harm.

Also, case shortening cannot be prevented by just neck-sizing cases so that the case shoulder is already up against the shoulder of the chamber when the case is fired. The primer has enough power to push NEW brass in a bit farther anyway, setting the shoulder of the case back.

Now, the interesting part is that it MAY be possible to work-harden brass enough (by firing enough full loads in it first) that the cases will not be driven forward enough by the primer in subsequent squib loads to cause any problems.

Still, if a person is going to use brass for "squib" loads, he or she is well advised to use a file or Dremel wheel to notch the rims of rimless cases, so in the future they are easily identified and are not used for full-power loads. Better yet, restrict "squib"loads to rimmed cases where there is no danger of excess headspace occuring from this cause.

BTW, there are long, detailed discussions of this phenomenon over on the 24 Hr. Campfire site, and numerous other gun-related forums.

quote:

Originally posted by Alberta Canuck:
Woods - The phenomenon of shoulder set-back which BartB. describes in his posts has long been well-known to occur with reduced loads, though it may not be caused by the strike of the firing pin. ...

Hey AC, I just rescanned Bart's posts and see no mention of "Reduced Loads". Looks like that should have been mentioned "if" that is what he was posting about.

It is an interesting point to consider, if Headspace on a Bottle Neck Case using Reduced Pressure Loads is created by strong Firing Pin Springs or robust Primers. Since you brought it up, there does seem to be some faint memories (40-50years old) about the need to Mark those Reduced Pressure Cases just as you said. At that time though, Reduced Loads were not of much interest to me and I stored that info in the Shorter Term Memory Retention areas.

I'll have to give this some more thought. I suppose a person could prove the Primer theory by using weaker than normal Firing Pin Springs to see if it still happens. Your thoughts?

And of course a person would have to forward all the Test Cases to the "Thingy King" to see what's happening with the Cases.

Nah, I'll just stick with my normal Loads, both Reduced and Safe MAX.

Amazing what a memory will retain.
-----

quote:

Originally whammed in by Bart:
Third, when the firing pin's released, it smacks the primer pretty hard; hard enough to drive the case forward setting the shoulder back a few thousandths; how much depends on firing pin spring strength, pin weight, shoulder angle and how smooth and slippery the shoulder mating surfaces are. It's usually in the range of 4 to 8 thousandths.

To quote a friend -

Best of luck to you.

quote:

Since you brought it up, there does seem to be some faint memories (40-50years old) about the need to Mark those Reduced Pressure Cases just as you said.

I have also come across that (more recently though ) and as a matter of interest, I actually found it to be so. But my idea of reduced loads are subsonic velocity with fast pisol powder which means the pressure hardly shows up on the primer. Both shoulder and necks seemed to set back. But - that was with a rimmed case! (303 Brit).

Woods, Canuck, 303Guy, Hot Core, others... been away a bit but came back to respond to your comments; some were excellent especially those on squib loads.

First off, the stuff used in Woods pictures show two things that virtually guarantee there'll be little or no shoulder setback in his tests. Shoulders on .280 AI cases have 40 degre angles and lots of area to resist shoulder setback more than most rimless bottleneck cases. And there visible evidence that one or two other things that might contribute to not setting a shoulder back. I don't know if Woods intentionally chose the specific hardware to get minimum shoulder setback or just didn't know how to run a test duplicating my original detailed post on what happens. He does and I'll let that issue rest.

I'd never read anywhere that shoulder setback from firing pin impact happens when I first had incidents of it. I ran the tests myself because I had some once-fired .308 Win. brass with their primers sticking out several thousandths. It took me a while to figure out what happened then I ran some tests with new cases starting with a max load then 1 grain less, then 2, 3, 4 and so on until the primers in those 1nce-fired cases started sticking out. The more the charge was reduced, the more they stuck out. A 15% less than max load is a significantly reduced load. Primer protrusion from new cases fired with reduced loads started happening when a 44 grain max load was reduced down below 40 grains, or about 10% reduced.

At my next match, I mentioned this to some folks and they asked me if I'd ever heard of "squib" loads as that's exactly what I'd experienced. The lights in my mind lit up upon hearing that remark; yes, I'd heard of and was aware of squib loads, but like all of us at times, I forget the obvious. That's when I ran my own primed case only tests on cases in .308, .30-06, .270 and asked others to check their .300 Savage, .358 Win. and some other cartridges to duplicat what happens up to the instant when primer's detonate, then burn the powder, peak the pressure and push out the bullet. The rimless bottleneck primed cases all set the shoulder back. Fired primers in fired cases have dimpled cups and crushed anvils that reduce the energy transferred to the case by the firing pin and don't test what happens with real ammo. Some cases had their sholders polished really smooth and shiny and after popping primers in them had microscopic rings around them from impacting the chamber shoulder hard enough to see the micro rings in the chamber shoulder. If cases weren't driven hard into chamber shoulder by firing pin impact, the "fingerprint" of the chamber shoulder wouldn't be imprinted on the case shoulder.

Using a .308 Win. rifles with both claw and sliding extractors, firing pins with the proper .055" +/- .005" firing pin protrusion and proper firing pin pre-load weight, both set back .308 Win. shoulders with primed cases. When weaker firing pin springs, and those with much less protrusion from the bolt face, were used, shoulder setback got less. One very old spring from a used rifle I bought to just use the action from was weak enough not to set the shoulder back at all, but it would fire live primers and shoot loaded rounds albeit at reduced accuracy (I'd bet its original owner kept the rifle cocked all the time when stored as evidenced by its much, much lower that spec preload weight). I'd just replaced the year-old firing pin spring with a new Wolff Blitzschnell 26-pound one on the rifle that had the issue happen but wanted to see what weaker, old firing pin springs did regarding shoulder setback upon primer strike.

Bart

You are misinterpreting the results of your tests. The firing pin itself did not have anything to do with setting the shoulders back other than igniting the primer. The primer ignited, the force of which pushed the case forward with enough force to set the shoulder back. If you had read AC's post and my response you would have picked up on this.

IOW, the primer explosion pushed the case forward because of the gases from the primer pushing the case on the inside of the primer cup.

Woods sez what I thought he would.....

quote:

You are misinterpreting the results of your tests. The firing pin itself did not have anything to do with setting the shoulders back other than igniting the primer.

One of my tests years ago was interpreted correctly, just like the test results I get these days. I just ran it again to see if the laws of physics had changed.

Two new .308 Winchester Super cases with Win. 120 primers, one with a live primer and the other's primer deactivated by water contact; a 'dud.' Both new cases had 1.627" headspace, about .004" shorter than chamber headspace of about 1.631". Primers were about .003" below flush with head. Here's the results after each were fired with an older firing pin spring with about 20 pounds of force at cocked/compressed length:

Live primed case, case headspace shortened .0035" and the primer protruding from case head about .008" no doubt caused by the pressure created from its detonation, chamber shoulder fingerprint on case shoulder. Note this case's dimension from shoulder to primer, about 1.631", is about the same as chamber headspace

Dud primed case, case headspace shortened .0035", primer .004" below flush, chamber shoulder fingerprint on case shoulder.

Where did the energy come from to set the 'clicked' dud primed case shoulder back .0035 inch; exactly the same amount as the fired live primed case? It had to be the only thing with energy stored in it; the compressed firing pin spring.