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.)