Can someone explain to me in simple terms the benefit of bumping your shoulders back when loading.
What makes it work?
Why does it need to be done?
How does it enhance accuracy, etc?
I don't know about you, but I sometimes do things that works (like bumping shoulders), and not necessarily understand the mechanics of it.
xphunter

well what a question. Thier appears to be many benefits of bumping the shoulders back. I am not sure what the truth is. While Bellm and some say it must be done. I do for the same reasons they state............MANY do not and only neck size and have Great results.
I would read all the info you can get and make up your own mind. Get BOTH sides of the story.
Like I said I keep the shoulders where the rounds headspaces with about .001 of the rim out of the barrel........ works with both of my frames ...But Many disagree and only neck size.
fireball

From everything that I've read, you want to avoid bumping the shoulder back. You want the sized case to fit the chamber as closely as possible. When you fire a round, the case expands to fill the chamber. The less room it has to grow, the better repeatability (i.e. accuracy) you will have and better case life. (Stretching and resizing cases will cause the brass to fail after X many cycles, resulting in splits and separations.)
This only applies to rigid firearms (front locking bolt actions, Ruger #1, etc). Contenders, Encores, and other less-rigid firearms will flex and stretch during firing. The best medicine for those is to measure the barrel-frame gap at the top of the breech when the barrel is closed and size the cases to extend out the back of the barrel just shy of that gap. For a much better explanation, go to Mike Bellm's website and read the articles he has written.

www.bellmtcs.com

Note: I am not a gunsmith, nor a professional in the firearms biz. I am an engineer who barely got a B in Strengths of Materials. But, having read and re-read the Speer and Hornady manuals, plus reading the advice of others, the above info makes sense to me and I can justify it logically.

If I'm off base, someone let me know.

Don't let other people make up your mind for you.
Including me.

Thanks,
Mike

Xphunter,

I have read alot about the benefits and flaws dealing with bumping shoulders.

Here is what I have seen with my own eye and know to be true on the matter.

As you know I am not an expert by any means just observing what my handguns and rifles have told me in the past.

Lets first cover the most common firearms we deal with, factory firearms.

I have found that once I have used my cases several times, neck sizing them only, eventaully they become much tighter in the chamber making it harder to lock the bolt or close the action on a neck sized round.

We all know this is because the more a case is fired the more it becomes work hardened and does not contract as much as a newer softer case would after firing. They expand and stay expanded.

We know this to be true of all firearms using metallic cases.

Well in factory guns, we all know very few are set up in a way that is beneficial to fine accuracy. Some will have bolt lugs that only contact at 60%, some have chambers out of plumb with the bolt face or receiver, or both.

With these firearms I have found it is very useful to bump the shoulder just enough to prevent a crush fit of the case in the chamber. In factory guns, this crush fit will magnify any problems in the set up of the action/barrel system.

For example if the bolt lugs are only supported by 60% of one lug, the pressure of the tight case will try to straighted the bolt and pressure it out of suare to the bore.

I have found that after about five firings and neck sizing, it is time to anneal the case as well as bump the shoulder a few thousandths for clearance with the chamber.

My factory firearms just seem to shoot better that way.

Now, when dealing with custom firearms built for accuracy, my view on bumping the shoulder is totally different. Since these firearms are put together as nearly as perfect as possible, at least we hope after writing the check for them, the ones I have all seem to prefer tight fitting cases.

For example my Win M70 with a 30" Lilja barrel shoots very well with fire-formed cases that easly chamber in the rifle, in the .5 moa range at 500 yards.

But that same rifle with cases that are quite tight, infact need a significant amount of effort to close the bolt, will shoot into .1 to .2 moa with the same exact load.

Keep in mind that both loads are are held to under .002" bullet run out and most are in the .0005" to .001" range. The only difference I have found is case dimensions.

So when these case become overly tight, I anneal them and size them just enough to slightly reduce the effort to close the bolt, there is still resistence but not as much.

My theory here is that since the rifle is built correctly and accurately, the tight fitting shoulder only holds the bullet in better relationship with the bore.

So as far as shoulder bumping goes, infactory rifles, I see a clear benefit from doing it but in custom guns, I see actual detractions from the process.

My LAR 50 BMG acts the same as my 6mm-284, as does my 338 WSM XP-100.

Like I said this is just theory, I am no expert, just listening to my rigs tell me what they like and do not like.

Good Shooting!!!

50

I set up ALL my full length to just touch the shoulder. It does not really bump it as it is more of a touch. I set up my neck dies to also touch the shoulder, very lightly. I have noticed that this consistently improves bullet runout. A friend has also tried this and confirms my results.

MSSMAGNUM (Mike) wrote: "I set up ALL my full length to just touch the shoulder. It does not really bump it as it is more of a touch. I set up my neck dies to also touch the shoulder, very lightly. I have noticed that this consistently improves bullet runout. A friend has also tried this and confirms my results."

This also pretty much sums up my approach to the situation. And, I've never encountered any problems with runout, accuracy or difficulty in chambering.

But I can also see where actually bumping a shoulder back, particularly where oversize (i.e. factory) chambers are concerned, may be beneficial as the "springiness" of the break-opens allows enough case stretch to create a crush fit if only the neck sizing approach is used. That would create difficulty in chambering, raise pressures and have a detrimental effect on accuracy.

I now have a particular batch of 7mm Bullberry brass that has gone through 16 firings and is currently due for only its third trimming. Each piece was sized so that the FL Redding die just touched the shoulder. The cases drop in with ease and simply fall out after firing. This is with plain-Jane, WW 30-30 brass that has been reformed, trimmed back to 1.750 and fireformed. And the load is not a wimpy one: 32 grains H335 under the Sierra 130 grain SSP.

XP, for your bolt guns, neck sizing is fine until you begin to have problems chambering rounds due to the diameters of cases being a too tight fit in the chamber.

And so long as you are not getting much flexing of the frame due to thrust of the case head against the frame, you can do well with neck sizing in the break open guns..... and no, fellas, I am not talking out of both corners of my mouth. The problem is that few folks know how far their cases stick out of the barrel and how hard they impinge on the breech face when closing the barrel. They just slam it shut a time (or two even) until it will fire.

The state of man has progressed beyond the stone age or fix it with a bigger hammer approach.... I hope.

In general, since most folks don't understand or know enough to actually measure their cases for protrusion, compared to the barrel-to-frame gap, it is easier to just nix the whole notion of neck sizing and size cases so that the shoulder to headstamp length FITS the gun. What a novel idea! Make it fit. Imagine that. What will they think of next?

That is in regard to the length of the case, and thus the actual "headspace," or lack of space.

Neck sizing done willy-nilly creates more problems than it cures and has confused and confounded more people than any other aspect except for understanding that the locking bolts have to engage the frame a minimum distance in order for the gun to function.

My approach is to tailor the chamber's diameters to very closely match the size die to be used. Ideally, the size die should always move the brass enough to always permit it to enter the chamber freely enough, BUT also bump the shoulder back AS REQUIRED without expanding the shoulder diameter and making cases stick, which is what happens if you bump the shoulders with a neck size die. I aim for not having a size difference between sized cases and fired cases greater than about .003," and much of the time it is only a thous. or two difference.

Working up to max to see what the maximum fired diameter will be.......then sizing the cases and comparing..... is the basis for determining the max. allowable pressure system with .444 Marlin cases that Don Shearer refined, btw.

Done this way, you have the best of all worlds. A case that is not too long to chamber without slamming the barrel shut, a case that does not stick because it is binding up going in, and minimal clearance between the case head and chamber wall for consistent alignment at that point. Combine this with a closely fitted chamber throat and bullets seated out into it, but not hard into the riflings and creating an overly long round again, where can the bullet go but "home," lined up with the bore? And with the Contender, you are not having the hammer block interfering with the fall of the hammer due to not being able to close the barrel all the way on an overly long round.

May all sound complicated, but it really isn't. Just make the cases fit the chamber. To do that, you need to be able to take certain measurements, such as barrel to frame gap and then measure the case head protrusion from the end of the barrel. Adjust that to be about .001" less than what the gap measures, and you should be in the "sweet spot." If not, then tweak the length one way or the other, but not by guess and by golly. Measure it and know what you are doing.

Mike

Now that I closed the above out.....the bottom line is to bump shoulders as required for a specific result, not just willy-nilly without regard to the effect produced.

Mike

I'm with Bobby..my FL Redding dies are backed way off to only Partially resize..I started with a case that wouldnt fit and gradually tightened down the die until the case was slightly("??) above flush. I dont think the shoulder is actually touched. And yes this Encore chamber is sloppy but here(pic) are some results out of a 24" barrle. This XBT is .075" off of my lands which would indicate to me that even with a sloppy chamber I am getting decent bore alingment using this resizing method.
http://www.imagestation.com/picture/sraid50/p1e911ce9a3bb1e57288cef0f80dbd3f0/fca85b19.jpg.
Dave

To all who posted,
THANKS!
I appreciate all the experiences, thoughts, and technical data shared.
Now, just to have more time at the loading table and shooting bench.
xphunter