Bullet Set-Back; Why Some Guns Go Kaboom

[SA Friday]

Someone asked in another thread what bullet set-back was. The question was answered accurately, but brief. I felt it would be a good subject for me to expand on in a new thread.

So, lets expand. How could a factory stock gun shooting factory ammo go kaboom? There are a few ways, but the first thing I ask about in these situations was how many times the round was chambered. Ya, there have been the 1 in 10 million double charged factory round, but you have a better shot at the lottery, IMO. Something stuck in the barrel? maybe, but that leaves evidence of the blockage. Almost always, it's bullet set-back that kaboomed the gun.

When the powder ignites in the cartridge, there is a spike in pressure. The bigger the space left after seating the bullet, the softer the spike. If the bullet set's back on chambering, the space shrinks and the pressure spike increases. If your bullet has set-back enough, the spike can be too much for the chamber, kaboom. Some cases and bullet combo's are more suseptable to this than others. I've found nickle cases can be the worst for it. The slipperiness that you so want for cartridge extraction will also make the bullet retention more slippery too.

So, if I have set-back issues, then I just load long and compensate for the set-back, right? Nope. Set-back is like poly-drug use. You might get away with it for a while, but eventually it's gonna get ya. The amount of set-back is completely unpredictable. The only way to avoid it is to not rechamber a round over and over again, and if you are reloading, use techniques to prevent it.

I've seen set-back problems happen with law enforcement at least twice in the last 11 years. Not all police agencies have the same experience level within their agencies when it comes to firearms. The typically send out a warning message to the rest of the LE community warning that this gun or that gun may have a problem. Both ended up being the ammo and not the gun.

Some cases have more of a problem with this than others. Pistol cartridges with straight wall cases are the worst. Rifle rounds can have this problem too, but it's seen in pistols more often. This is because rifle rounds are not rechambered as often. I have seen military 5.56 have set-back problems while in Iraq. In these cases, the bullet ended up so set-back it obviously shouldn't be fired. With a pistol, you might not catch it unless you are looking for it.

Set-back can be a serious problem for new reloaders too. You should always test a new load combination for bullet set-back. The easiest way and the one I use, is to make a dummy round and press your thumb against the headstamp while the nose is pressing up against the side of a bench. If you can move the bullet before your thumb hurts, you don't have enough tension to stop bullet set-back when you chamber the round.

Well, shit, what do I do now, the bullet set-back on the thumb test? More crimp is usually what people do. It works, but I don't like to crimp my semi-auto pistol rounds past just taking the bell out of the cartridge. It's not the best resolution. I have switched to a different type of resizing die for my semi-auto pistol rounds now; an EGW Undersized resizing die, commonly called a 'U' die. It resized the case .001" tighter than SAAMI specs. This stops set-back.

How do you know the U die stops set back? Have you tested it? Well, hell yes I did. That's what I do. I went down into my cave and tapped into my armada like arsenal for some stuff to test. 40 S&W is the cartridge bullet set-back seems to happen the most from what I've seen. It has all the right parameters: straight wall case, higher SAAMI chamber pressures, rechambered in the gun a lot for CCW and LE alike.

I resized two cases, one using a U die (carbide) and another in an RCBS carbide die. (Picture 1: RCBS on left, U die on right)

(I'm gonna add additional posts to the thread with pics as I go so hold off posting so it will flow, please.)

[SA Friday]

I then loaded them with the same manufactured bullet, a 180gr moly coated lead bullet. I then measured the OAL of both rounds. They didn't need to be the exact same length for the test.

[SA Friday]

The are not loaded to normal length for 40 S&W. They are loaded long. My seating die is set for my STI's, and they prefer longer rounds. As it so happens, this makes the results easier to see.

So, I then chambered the two rounds in my STI Edge. I used my Edge because it has a lightened slide and only uses a 10lb recoil spring. It chambers very soft. If a bullet isn't going to set-back, it's going to be in this gun. I measured both rounds after being made, then after being chambered once, 5 times, and 10 times measuring after each. This is what I found:

RCBS resized: 1.183 OAL, one chambering 1.179, 5 chamberings 1.165, 10 chamberings 1.132.

U Die: 1.188 OAL, one chambering 1.188, 5 chamberings 1.187, 10 chamberings 1.181.

The RCBS round lost .018" after 5 chamberings, and .051" after 10 chamberings.

The U die round lost .001" after 5 chamberings, and .007" after 10 chamberings.

I highly doubt .007" would make any difference in chamber pressure. Most reloaders have more varience in their seating depth than this for pistol rounds. A .051" difference will probably have bad ju-ju consequences.

What about factory ammo? I just so happened to have some of that stuff, no it's not for sale. Winchester 165gr FMJ target stuff, commly referred to as Winchester White Box. I used the same gun:

WWB: 1.121 OAL, one chambering 1.121, 5 chamberings 1.121, 10 chamberings 1.120.

(Wait for it, there's even more info coming.)

[SA Friday]

OK, what about a non-wussie competiton gun? What will a Glock 23 with factory recoil spring do? Mmmm, more testing. Lets see. I used a factory WWB round for this:

WWB in G23: 1.120 OAL, One chambering 1.120, 5 chamberings 1.117, 10 chamberings 1.113, 15 chamberings 1.108, 20 chamberings 1.103, 22 chamberings 1.100. The round failed to feed on the 23rd chambering. It was too short at that point.

Somewhere in between 10 and 15 chamberings, things could get pretty hairy.

Cycle your ammo if you load and unload your gun often. DON'T continue to reload the same round. If it looks short in comparison to the other rounds, it is. Chuck it. Soak it in a cup of bleach (WD-40 doesn't work) and throw it away. You shoot off the ammo and replace it? That works too. Just don't disregard bullet set-back. It would suck big time if your first fired round in self defense blows the gun up.

(K, I'm done now...)

[Irving]

I have a question then. On the occasions when I do need to unload my gun, I usually unload the mag half way (all the way after that other thread) and drop the previously chambered round some where else in the mag (last round now). I put it in the last round so if the gun is going to have a failure to feed, it will be as close to the end as I can get it. This seems like it might not matter that much though, because I very rarely end up re-chambering a round at all, let alone more than once.

So what I'm saying is, moving bullets around to avoid rechambering is kind of silly, since that bullet is going to be rechambered again when it is fired anyway, so if I'm not worried about it blowing up because it will have only been chambered one other time, why should I worry about it's position in the magazine at all? I mean if I chamber a round, then have to unload the gun for some reason; no matter where I put the bullet back, I will always have that one bullet that is going to be chambered twice no matter what. So why not just make it easier on myself and rechamber that round without having to swap out the whole magazine?

Of course, I still won't do that because it is just setting myself up for making a bad habit, but you understand what I mean right? If you are worried enough about chambering a round more than once that it makes you feel better to move it, but you don't think it will explode due to over pressure, then there is no reason to move it at all. It sounds like the most safe practice is to just throw away any rounds you don't trust right?


Good post by the way. Have you tried to fire a round that has been soaked in bleach? Just to make sure it works?

[MuzzleFlash]

For rifles especially, under seating can cause pressure spikes too. This seems highly counterintuitive since you start with an excess of case volume. The problem has to do with overcoming engraving pressure. When the bullet is set back a few thousandths from the rifling, it gets a running start at the rifling and begins engraving with plenty of momentum and pressure already behind it. If seated too long, the bullet is in contact with the rifling and needs higher pressure to overcome a much higher initial resistance. This almost always results in over pressure. Competition handloaders have to balance the desire for higher accuracy obtained with tight chambers and longer C.O.L. against the danger of excessive pressures.

An intuitive example of this phenomenon would be to back your car out of one of these surburban driveways with the continuous curb that cheap ass developers love to install. Aim the car straight at the driveway. Now put it in neutral and have a buddy push it with a running start at the driveway curb. It's a lot easier to overcome the curb with a running start than if the front tire is already resting on the curb when he begins pushing.

Hornady has a very good write-up on headspace and other internal ballistics issues on their web site:

http://www.hornady.com/ballistics/internal.php

[Irving]

Wow, that is some good info you just posted. SAFriday, you've been starting some great technical threads on here lately.

[SA Friday]

I have a question then. On the occasions when I do need to unload my gun, I usually unload the mag half way (all the way after that other thread) and drop the previously chambered round some where else in the mag (last round now). I put it in the last round so if the gun is going to have a failure to feed, it will be as close to the end as I can get it. This seems like it might not matter that much though, because I very rarely end up re-chambering a round at all, let alone more than once.

So what I'm saying is, moving bullets around to avoid rechambering is kind of silly, since that bullet is going to be rechambered again when it is fired anyway, so if I'm not worried about it blowing up because it will have only been chambered one other time, why should I worry about it's position in the magazine at all? I mean if I chamber a round, then have to unload the gun for some reason; no matter where I put the bullet back, I will always have that one bullet that is going to be chambered twice no matter what. So why not just make it easier on myself and rechamber that round without having to swap out the whole magazine?

Of course, I still won't do that because it is just setting myself up for making a bad habit, but you understand what I mean right? If you are worried enough about chambering a round more than once that it makes you feel better to move it, but you don't think it will explode due to over pressure, then there is no reason to move it at all. It sounds like the most safe practice is to just throw away any rounds you don't trust right?


Good post by the way. Have you tried to fire a round that has been soaked in bleach? Just to make sure it works?

You could do that. Just use the same round, and measure it every couple of chamberings to see if you have set it back. When bad, toss it and get another.

I have bleached rounds. I got that info from a chemist that used to make primers for a living. Oil based products won't break down the chemicals in the primers permanently . Bleach will kill the primers and the powder.

[Graves]

This has been my take on the kaboom theory ever since a trip to the range a while back. I cc'd into the range and fired a few boxes through a few of my pistols, then I decided to pull my carry gun out for a few rounds. Not wanting to spend any of my 9bple down range I racked the slide to remove the chambered round and I was shocked from what I saw, the round I carried chambered was had some major set back. I don't think the bullet could have been pushed in any further, and I know that particular round had only been chambered once. I am sooooo glad I decided to run some ammo through my carry gun that day.

[Irving]

Y
I have bleached rounds. I got that info from a chemist that used to make primers for a living. Oil based products won't break down the chemicals in the primers permanently . Bleach will kill the primers and the powder.

I want to make it clear that I believe you about the bleach, I'm just curious if you've actually tried firing bleached rounds before. I think it is something I'm going to try next time I go to the range. I don't know anything about how primers works, so sorry if this is a dumb question; but is it possible to only weaken a primer with the bleach? Like if you don't let it soak for long enough or something? I don't want to try and fire a bleached round and end up with a round stuck half way down the barrel.