A caution for those who load things like the 40S&W and 367sig. These are especially prone to problems of bullet setback if you chamber the round a few times . The 40 for example will double the pressure if the bullet sets back .10" That's 35,000 raised to 70,000psi !!!

I've assumed that seating a bullet deeper into the case would increase pressure. Somewhere, sometime, in a galaxy not so for away(possibly this forum), I read a statement that seating the bullet deeper would actually decrease pressure. The reason given was the bullet travels farther
before it is(for lack of a better word) jammed into the rifleing, thus allowing peak pressure to
ocurr before, or soon after, the bullet meets the resistance of the rifleing. Question 1: Anyone tell me where I may have read this philosophy? Question 2: Anyone tested seating depth vs. velocity? Thanks, Jimmy

You could've read it in several places where I've seen this topic discussed recently. You've summarized the main points I've seen, and I think that for any specific case you'd just have to test it empirically to know exactly what effect it had. There are some places with data on the Web that I've seen, but don't have bookmarked. Saw one graph of pressures and velocities showing the chamber pressure about 6,000 PSI higher with the bullet touching the lands vs. some small distance away. I don't recall the cartridge or load. Someone else may have links handy. You'd have to seat the bullet pretty deeply with most rifle cartridges before you decreased the powder capacity enough to start building the pressure back up that much. Check the threads about problems with Weatherby Magnums chambered in custom guns with short throats to see what effect a long "freebore" has in reducing pressures. Many custom rifles without the standard long throat can't fire Weatherby factory ammo safely.

I would make a correction to the way you explained the lower pressure with the bullet seated farther from the rifling origin: It's not because the peak pressure occurs earlier, which would RAISE the peak pressure. It's because the bullet starts moving faster and delays the peak pressure point so the bullet's farther down the bore and the powder has more space to expand in. The bullet gets a "runny-go" at the rifling and swages into it faster than if it starts off pushed against it. Try pushing, say, a shopping cart over a door threshold when you start from a standstill with the wheels right up against it. Now push it from a foot away. Which is easier? Same effect.

Seating deeper will increase your pressure. Rifles like the Weatherby's have allot of "freebore" or a long throat that allows them to handle hotter loads, because the bullet has to "jump" farther to the lands. It doesn't do much for accuracy though. That may have been where you heard the longer jump to the lands ='s less pressure.

if you had 2 rifles, that had .0275 leade, after the bullets are set, and the barrel were IDENTICAL (can't happen) the one with the, call it, 3.00 OAL with the .0275 leade would have LOWER pressure, same load, as the 2.85 with .0275 leade.

it's chamber pressure for a reason.. make the chamber smaller, pressure goes up.

jeffe

True. But the chamber volume reduction will be an extremely small percentage, and in an actual rifle the leade increases by the same amount your OAL decreases as you seat the bullet deeper. It's not as simple as you suggest.

This is interesting, and I have often wondered the same thing.

It seems to me that seating the bullet farther and farther out (holding load, temp, throat (aka freebore), leade, etc. constant) will steadily DEcrease the pressure. However, when the OAL reaches the point where the bullet actually touches the riflings, the pressure will jump dramatically - a point of discontinuity in the pressure curve.

The claim was made that deeper seating depth will DEcrease pressure, which builds, I presume, on the idea that the bullet can get a "running start" at the riflings. However, this seems to imply that the bullet will be ACCELERATING down any free bore. For if it were not accelerating, it would not be getting any better a "running start" if the bullet started 0.2" from the riflings or from 0.002" from the riflings. Hmmm....

Another possibility is that all loads that do not touch the riflings, will have the same pressure (all else constant) because the "perimeter" of the chamber is not the end of the loaded round, but rather where the riflings start. In other words, pressure won't build (to operating pressures) until the bullet contacts the riflings. So whether you start 0.2" off the riflings or 0.002" off the riflings wouldn't make any difference. This would be consistent with why fewer grains of powder are need to drive heavier (and longer) bullets to maximum pressure; i.e., there is a smaller volume for the combusting powder to build pressure.

Anyway, this is all just brainstorming. Neat thread though...

I have to agree with Ricochet on this one.

Peak pressure ALWAYS occurs when the bullet engages the rifling. Weatherby's long throat allegedly reduces the this peak pressure some, by allow the bullet a "running start" at the rifling--and as pointed out--at a cost of accuracy. The start up in pressure gets the bullet moving long before peak pressure occurs.

At the other end of the spectrum is most benchrest shooters seat the bullet jammed in the rifling ("against the jam") a quicker rise in the pressure curve results and current wisdom says the bullet against the jam is essential to accuracy--IN BENCHREST RIFLES (DON'T DO THIS AT HOME UNLESS YOU HAVE A REAL GOOD IDEA OF WHAT IS GOING ON!).

There are a number of reports (please don't make me dig through my files and cite them this time of night) that indicate the bullet for all intent stops for a miilsecond or so when it "hits" the lands, before continuing down the barrel--this is where peak pressure occurs regardless of how far from the rifling the bullets starts from.

I am giving serious consideration to testing some loads in a .243 hunting load/rifle with the bullet against the jam verses the bullet seated (.02 or so from the lands) where I THINK I am currently getting the best accuracy. Of course I am going to start at the lower end of the powder charge!

Casey

Mete

Very interesting point.
I reload .40 S&W cartriges for a 1911 at Overall Length : 1.25",(instead of 1,135" specified for .40S&W).
Is there any calculation for inferring pressure decreasing?

BA Shooter

I don't remember what was concluded, but I think a fellow who went by the name of OKShooter did a pressure test regarding bullet seating distance. His web site is http://www.reloadingpro.com/
I'm sure he could clear this up a little better for us all.
John

I put up a post on this a while back. From what i concluded with a freebored round like 300RUM, there is a window of operation where pressure can be min. In other words if you are at the Lands pressure might spike, but the lower you seat pressure will go down and then you reach a point where pressure starts going back up the deeper you seat. The way one guy at Sierra tech support explained it, its a give and take thing. Yea you are giving the powder more room to burn, but less of a jump to open up the chamber, but the deeper you go your giving less room for initial ignition for more room for the bullet to jump. So its a compremise. I don't have any data handy but did take some velocity and case expansions to verify.

Generally speaking, the deeper the bullet is seated the higher the pressure. This has been my experience anyway, and it did not matter if the throat was a taper or a cyclinderical style.



To know for sure you need to test it in your rifle.



The location of peak pressure is a function of the actual powder being burned. In MOST cases the peak pressure will occur between 10% and 20% of the way down the barrel. The peak pressure should NEVER EVER occur at the instant the bullet engages the rifling! If so you are using too fast of a powder or too large/hard of a bullet!



Scott

John, thanks for posting the link to OKShooter's site! I appreciate his experimental approach. I have his .30-06 manual, and recommend it. Hadn't talked to him in a while and needed to look him up to ask a question or two.

Jeffe, I really don't want to argue with you and certainly don't intend to make personal attacks ("closed mind," etc.) I agree with your statement that shortening the OAL with the bullet-to-land distance the same would increase pressure. I'm merely pointing out that the question applies to seating a bullet more or less deeply in the same rifle, in which case the distance to the lands does change. That's all. It's pretty hard to control other small variables between two different rifles; a valid direct comparison would be difficult.

BAShooter, somewhere on the internet (search 40 S&W) there is info on seating depth vs pressure for the 40, though I don't think it included less than normal seating. This all came about from the Kboom incidents. They recommended that handloaders not use the 180 gr bullet for that reason.

I think everyone just about has it straight that there is a bath tub shaped curve of pressure starting where the bullet is jammed deep in the case and ending where the bullet is jammed in the riflings.

I have done lots of pressure experiments, and I would like to add two things:
1) Bullet set back or jammed in the riflings will cause a small increase in pressure compared to the bullet being pinched radially.
2) The variable of how much unburned powder is blowing out the muzzle in a straight wall cartridge is why I can shoot so many pistol cartridges with loads that Quickload estimates as over 1,000,000 psi, and yet the primers look ok. LIL'GUN in 38 Super in one region decreases velocity with increases in powder charge.

So in my project to build a map of what it takes to make the primer fall out in 36 different calibers, I JUST WISH I could set the bullet deeper .1" and double the pressure. What can happen is more powder or deeper seating can cause some radial pinching, and too often the person collecting the data does not go back and find the real cause of the pressure spike.

--
A society that teaches evolution as fact will breed a generation of atheists that will destroy the society. It is Darwinian.

Scott, and others,

I referr you to:
http://www.shootingsoftware.com/pressure.htm

While looking (hoping?) to "invest" in my own ballistic lab, I ran across this website. This is the RSI/CED website advertising the Pressure Trace setup (the best competition to Oheler Ballistics Lab). Approximately halfway down the page, the second graph (6mmPPC) shows different effects on pressure with seating depth. The graph shows pressure curves with the bullet seated to engage the lands and with the bullet seated 4 thousands off the lands. Although it does not specifically state as such, in both cases the peak pressure is occuring when the bullet is initially engraving into the rifling. Also, note peak pressure occurs .25 milliseconds-.30 milliseconds into the pressure graph--entire duration of curve is 1.25 milliseconds-1.40 milliseconds long. Peak pressure is occuring very early in the pressure curve. Primer ignition, ignition of some of the powder, some amount of powder cumbustion, all must occur before peak pressure--that eats up most of the .25-.30 milliseconds. The neck (along with the rest of the case) begins to expand--and releases the bullet The next step, is when the bullet is beginning to engrave (and be compressed) into the rifling (Remember, if the bullet is seated away fom the lands, it virtually stops when it first "hits" the lands). Peak pressure may occur just as the bullet becomes fully engraved--and that can vary some on individual rifles. But in either case, this is still part of the initial engraving process.

There are several more websites I have ran across that demonstrate this. Also, I believe Hatcher's Notebook, (among others) mentions this.

But I must admit my choice of words ("ALWAYS") is something that I should not use when speaking about the limited info and knowledge of ballistics (including my own limited knowledge). In other words, there may well be an occasional set of conditions where peak pressure occurs after the bullet engages the lands and has moved some distance down the barrel--but I still think it would be rare.

Casey

CaseyC,
If I go to:
http://www.shootingsoftware.com/pressure.htm
They show a peak ~.3ms after the instrument triggers.

In Quickload, if I specify, 6mmppc, 70 gr. TNT, 25 gr. H332, I get 35kpsi with the peak of pressure occurring in 1.079 ms and 1.70" of bullet travel.

Quickload thinks it takes about .15 ms to get up to 10kpsi, about where the Recreations Software trace begins.
If I add those times; .15ms + .3ms = .45 ms does not equal 1.079 ms.
So we have some disparity over pressure rise times.

--
A society that teaches evolution as fact will breed a generation of atheists that will destroy the society. It is Darwinian.

Peak pressure never occurs right at the time the bullet hits the rifling, as that would mean you are using too fast of powder or having a detonation, either of which dismantles
regular firearms.Peak occurs about 1.5 to 3 inches of travel,depending on powder, bullet wt and hardness, temp,
and combustion chamber shape.Ed

Clark,

I think we are saying the same thing about the data, but our conclusions differ.

It is my understanding, in the strain gauges used by RSI/Oehler, the time (X axis) is still triggered by shock waves. This can occur with primer inginition (remember, shooting a cartridge loaded only with a primer can still force a bullet well into the barrel). Also, the rise to 10,000psi can occur with primer ignition and very little powder ignition/combustion (I don't disagree with Quikloads estimate of .15 milliseconds). Also, the location of the first sending units attached to the barrel are approximately midway point of the chamber (right in front of the barrel/action mating point). This all leads to a lag time.

So I was wasn't using "voodoo math" to to suggest: ".15ms + .3ms = .45 ms does not equal 1.079 ms". You are right--it doesn't. Quikload is estimating bullet travel based on estimated time--not where the bullet is located in the barrel/chamber. Also, the highest pressure is occuring right at the base of the bullet most of the time (there are different areas of pressure occuring inside the chamber/bore at the same moment in time). Remember, the nose and ogive of the bullet doesn't count, it isn't until the majority of bearing surface is engraved into the rifling do we see peak pressure. Giving the distance of the bearing surface of the bullet, and the taper of the throat, to the actual bore dimension of the specific caliber--it is at this point where the bullet stops (or almost stops) and peak pressure generally occurs. This may very well be 1.7 inches of bullet travel if we measure from, say midpoint of BEARING surface when bullet is at rest, to where the bearing surface has become engaged enough to where the bullet stops momentarily.

This subject is one that can be difficult to articulate at times, I hope I explained my thinking on this.

Casey

CaseyC,

Thank you for the link. I was commenting from my own experiences. The question asked, I understood to be general in nature, so I responded likewise. I think you will find significant pressure / distance curve fluctuations from one powder type to another, and with the differences in bearing area from bullet design to another, etc, etc, etc. I can find documentation that supports my statements as well.


In truth, the gentleman who asked the question, needs to test bullet seating depth variation in his rifle, with his powder, and his bullet. Only then will he know for sure.

Scott

Go to http://www.shootingsoftware.com/pressure.htm

About halfway down the page are some pressure traces that illustrate the difference between seating at the lands and .030" off the lands.