Bullet setback, Part II

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Jim K

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Two weeks ago I posted on some experiments with bullet setback, supposedly a common cause of guns, especially auto pistols, blowing up. Bullet setback occurs, according to the theory, when repeated loading of a single round (like the top round in a magazine) causes the bullet to be forced back into the case by the feed ramp, or the recoil from firing of the first rounds in a magazine causes the bullets of later rounds to be forced back. In either situation, the bullet being seated deeper in the case than normal will, supposedly, cause extremely high pressures and will very often cause the gun to blow up.

At that time, I reported experiments with 9mm Parabellum, which basically showed that there was indeed an increase in pressure, as best I could judge without pressure testing equipment, but that I did not think the pressure rise was enough to blow up a gun.

This week, I repeated the experiment with two other calibers, .40 S&W and .45 ACP. I used factory loads throughout, to reduce/eliminate the possibility of a double charge and because the variety of handloads is almost infinite. As with the 9mm, I used two different brands of ammunition in each caliber.

This is what I found.

.40 S&W

The cartridge overall length (COL) of both brands (Winchester and Independence) was 1.123"; two cartridges of each brand were fired from the box as controls.

Using a vise, I pressed the bullet into the case to obtain a COL of 1.097" for two cartridges of each brand. I repeated this to obtain a COL of 1.047" for two cartridges. For a third pair of cartridges, I pushed the bullets in as far as they would go. This was 1.005" for one brand, and .958 for the other. The bullets were seated on the powder charge and the difference apparently was in the amount of powder. I fired all the rounds in a S&W .40 Sigma.

The results:

The control cases showed normal primers and no head expansion.
The cases from the 1.097" rounds showed normal primers and no head expansion.
The cases from the 1.047" rounds showed flattened primers and some extrusion into the firing pin hole, but no head expansion.
The cases in which the bullet was seated as far as it could go showed primer extrusion into the firing pin hole and head expansion of .003"-005". That is a sign of significant pressure rise, but not, IMHO, enough to blow up a gun or even burst a case given normal support.

Again, while deep seating a bullet can cause a pressure rise, I doubt that bullet setback alone can cause enough of a pressure rise to result in blowing up a gun, at least with factory loads. I might also point out that the extreme bullet setback of the latter situation is impossible to conceive as happening accidentally. It took hard, deliberate pressure on the vise handle to get the bullets that deep and the cases were bulged.

.45 ACP

Again two brands were used, GI WCC and Hungarian Lake City, both GI spec loads. COL of both was 1.253". The first setback was to a COL of 1.180"; the second to 1.167". As in the .40 experiment, the bullets of the last four rounds were pressed in to seat on the powder with COL's of 1.044" and 1.074" respectively. All rounds were to be fired in a Norinco pistol.

The results were somewhat different. The control rounds and the first two setback rounds showed no sign of high pressure; primers were normal and case heads showed no expansion at all. I was unable to test the rounds with the bullets seated all the way in because the seating bulged the cases to the extent that they would not chamber.

So, again, the results are not really conclusive. I have not proved, absolutely, beyond any doubt, that bullet setback will not cause a blown gun. But I believe that I have proven that bullet setback will not ALWAYS cause a blowup, and it is my opinion that automatically placing the blame for high pressure on bullet setback may not be valid. I think it might be more productive, at least when dealing with reloads, to consider the more usual causes of high pressure, like an excessive charge or improper loading.

Jim
 
I have not see the full report of the Hirtenberger tests only summaries .Obviously the 'double 'the pressure number got my attention.Variables include bullet weight , powder ,chamber etc. I'll be carefull !!! Thanks for the info.
 
Powder differences should really matter. A fast burning powder and perhaps a larger, slower moving projectile would allow for higher pressures to build up.
 
I have a Para Or p-16 in .40 S&W. I also purchased a high-end aftermarket barrel in .357 Sig and installed it. The PO has a ramped barrel and the ramp is rather steep. I've had no problems with the .40 but the .357 caused bullet setback sometimes when it feeds from the mag. The .357 being a high pressure round, I was concerned so I kept an eye on things closely.

Even with the setback in the .357 round (maybe helped along by the short neck of the round not holding the bullet securely), I have noticed no evidence of increased pressure or other problems.

Of course, anecdotal evidence like this DOES NOT apply to any other gun or curcumstances.
 
Jim,

What bullet weight did you use in the .40?

Also, would you say that these loads were already near max, or were they pretty mild?
 
Both .40 loads were 180 grain, FMJ. Not much diversity, but that is what the stores had. I agree that powder charges and bullet weights could, and probably would, make a difference. But I was testing one point only - whether bullet setback alone can cause extremely high pressure. My conclusion is that it would not.

Could I come up with some powder/bullet combination that would? Certainly a double charge would do it, and quite possibly some otherwise innocuous load would do it with bullet setback, which is why I cannot rule out setback as at least a factor in high pressure problems. But the statement has been made that if there is any bullet setback, in any caliber, to any extent, the gun will blow up. If that were true, my guns would be scrap, and I can assure you that they are quite well.

Jim
 
Mr. Keenan,

I regard the 'setback' issue to be limited to internet board discussion.

I cannot "limp wrist" a gun that I own.

I can chamber the same factory round hundreds of times without 'bullet setback'.

I can shoot many round of +P+ rounds in any of my guns without 'blowing' them up or leaving any wear that can be measured by visual inspection or a micrometer.

If I choose to change the OAL length of a pistol or rifle reload, this does not induce 'setback' that will blow my gun up, skeer the wimmin n chilrrun and make the chickens quit laying.

More cybergarbage at the speed of light.

Wonder what the next myth will be?
 
But I was testing one point only - whether bullet setback alone can cause extremely high pressure. My conclusion is that it would not.
That seems to be a more sweeping conclusion than is perhaps warranted...

FMJ ammo tends to be "practice" ammo and is often loaded pretty lightly. SD ammo is often loaded to maximum pressures. My understanding of setback pressure increases is that they're not linear--that is, a given amount of setback doesn't raise the pressure the same amount. The pressure increase is also dependent on where you are in the pressure curve to begin with.

I'd recommend you repeat the test with 180gr SD ammo, but I'm not sure it's advisable...
But the statement has been made that if there is any bullet setback, in any caliber, to any extent, the gun will blow up.
I've never heard any statement like this from a credible source. Not only are the effects of setback very caliber dependent, they're also heavily dependent on the loading within a given caliber. In fact, I've only heard of one specific caliber and loading (.40S&W 180gr) that's been implicated as being setback sensitive enough to cause pistol blow-ups.

Even in that caliber & loading, from everything I've seen, it takes a significant amount of setback to cause problems.

To test this properly (assuming I had the time & money), I'd break it into three parts.

1. Using several common SD loads (and making sure to include at least one or two 180gr .40S&W loads since that seems to be the loading that sparked this controversy) , repeatedly rechamber a single round in a couple of common pistols and chart the setback resulting from the repeated rechamberings. This eliminates any questions about what levels of setback are reasonable.

2. Once you've determined reasonable amounts of setbacks that can be caused by a practically probable situation, evaluate the effects of the setback using reloading software to estimate pressures.

3. If the pressure estimates do not indicate that the results are likely to be dangerous, test the setback rounds from step one in a couple of common pistols for that caliber.

Where's Clark when you need him? :D
 
Jim;

I'm very interested in your test and results. I also agree with JohnKSa that perhaps 180 gr defensive ammo is more relevant.

What pistol did you test this ammo in? How would you rate the case support? Glocks, of course, are known for having minimal case support. Did you fire these pistols remotely?

-John
 
Jaysouth;

I, for one, have personally discovered two rounds, both .45 acp and either Remington or Winchester, that were set back from use.

A buddy of mine is a rangemaster at a local police dept. Every 6 months his department qualifies. The officers shoot up their duty ammo in the qualification, and then put the remaining rounds, if any, in a bucket before reloading their mags for the next 6 months with fresh duty ammo.

My buddy and I, in the course of our shooting, have used some of this surplus ammo for general shooting. I have discovered the two setback rounds in this ammo. Upon examination, the rounds show evidence of repeated chambering and extraction. Likely, these rounds were from the pistols of a "gun nut" who frequently practices with his duty weapon. This officer would eject the chambered round of duty ammo and empty his/her mags and load them with practice FMJ ammo. At the conclusion of the practice session he would reload his mags and chamber the round again. What's interesting is that in each case I observed, the setback round was not in the chamber when the qualification occurred (otherwise it would've been shot).

Now, as to whether it matters, I don't know. I'm not taking any chances.

-John
 
My carry load is a hard cast 225 gr TC over 6.5 grs of Unique. I load this to an OAL of 1.250. Most of the time I carry these loads in a Colt or Norinco 1911. This is a hot load that is very accurate.

If I suddenly decide to load to an OAL of 1.20 so the loads will work well in my Sig 220, is someone telling me that the gun will blow up?
 
I think squishing a 40 S&W round down from an OAL of 1.123" to 0.958" is asking for trouble. You only did this on one round though, right? I don't think that is enough of a test to say anything other than, as you said, bullet set-back won't ALWAYS cause a problem.

I think if you would run that same test on 100 rounds of ammo you might have a different opinion, but I would not encourage you to do that. Losing eyes and fingers probably isn't worth it.
 
Hi, guys,

"I'd recommend you repeat the test with 180gr SD ammo, but I'm not sure it's advisable..."

I will do so as soon as I can find some. I used what I could get. On four rounds, I pushed the bullet down as far as it could go; due to the different powder charges, one brand went to .958" OAL, the other to 1.005". As I said, I used an S&W Sigma. fired from the hand.

As I reported, there were definite signs of high pressure with the four shortest loads, but nothing blew up, even though conventional wisdom says it should have. Even if high pressure did result with an OAL of .958" so what? How could anyone think that multiple chambering or recoil could set a bullet back that far? I had to put a lot of pressure on the vise handle to FORCE the bullet down tightly on the powder, and the cases were bulged from the bullet being forced down into the thicker part of the case.

Could that happen just from repeatedly feeding a round? Of course not. The cartridges of 1.097" OAL showed no signs of high pressure; those of 1.047" only some primer flattening.) The more I hear on this the more I am convinced it is a myth, at least under the conditions usually given.

Jim
 
If I suddenly decide to load to an OAL of 1.20 so the loads will work well in my Sig 220, is someone telling me that the gun will blow up?
Absolutely NOT! The only reasonable data I've seen on setback indicates that:

  • Different calibers have widely varying sensitivities to setback.
  • Even a caliber that is sensitive to setback will probably only be sensitive to setback with the bullets on the heavier end of the spectrum for that caliber.
  • Even in sensitive loadings, it takes more than 0.05" of setback to cause problems.
  • Even with the "right" caliber, "right" bullet weight and "right" setback, you're not likely to have problems unless the round was already on the hot side.

There's no question that reducing case volume while holding other variables constant (an apt description of setback) will increase pressures, how much depends on a lot of things.
 
At one time, I thought I had to have a 357 SIG Gov't model, so I re-barrelled my Colt Delta Elite. Thus began an Odyssey of reloading problems!

There are those who believe that bottle necked cartridges such as the 357 SIG feed more reliably. In the cases of the 2 SIG's which I have owned and had barrels for both 357 SIG and .40 S&W, this has not been true. The 357 SIG has fed well, and so has the .40 S&W. Now, in the Gov't model everything gave trouble except the .40 S&W. After much work on the extractor, 3 different barrels, recoil and main springs, and a small radius firing pin stop, plus a different magazine catch, dumping a couple of magazines, and much experimenting with cartridge LOA and bullet profiles, it works.

During the course of this experimentation, I would suffer failures to feed in 357 SIG. Invariably, there was bullet setback, some times quite severe. I am sure that some of the cartridges which chambered and fired had a LOA which was substantially below what I had originally loaded.

No cases blew, ever. I am not saying that with a maximum load in an already weakened case that the 357 SIG will not blow a case head when the bullet sets back. I am just saying that I have not observed that effect so far.

The Lee FCD die in 357 SIG is the best thing I have found since sliced bread and bottled beer, period. It works.

Bullet setbacks might explain blown case heads, but in my experience, not catastrophic failures.
 
So the conclusion is that seating (or forcing) a bullet at or below...

C.O.L. (compressed overall length) causes pressure increases? I believe that's what any good reloading manual will tell you right up front.
Is the conclusion that slight variations past C.O.L. may or may not blow up your weapon?
I'm really not questioning your data or your conclusions. I'm just wondering why you've undertaken the task. It seems like this is all common knowledge.

If I've missed something, please forgive me.
 
Just to throw another loophole in there, what gun were you using on the .40? You might not see enough force to destroy a gun with a fully supported chamber, but switch to a Glock .40 with a non-fully supported chamber and see if your results are different.

I wouldn't be suprised if they were.
 
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