First, let me say, I do not want to fan any flames, simply wish to pass on some information which perhaps may help someone avoid a dangerous slam fire.
I have put a lot of rounds downrange through M1 Garands & M1A's but not too much experience with the AR's and none w/AK's, so following refers to the M1 types.
Other than a dirty chamber, a broken firing pin or a pin that is no longer free floating for whatever reason, IMHO, ammo is probably the number one cause of slam fires. Anything that can cause the round to "stop short" of full chambering can result in the firing pin hitting the primer with sufficient force to set it off. If this occurs before the bolt is in battery, it can be disastrous!
Soft primers can, as pointed out by Freightman result in slam fire and the CCI #34 military primer has a hard cup to duplicate GI ammo, most of which is loaded with a hard primer. Handloaders must be aware of two other factors in order to avoid slam fires.
First, make it a practice to run your thumb over the primer as each round is removed from the press to be sure that the primer is fully seated.
Secondly, each cartridge must be sized sufficiently to fit chamber giving proper headspace clearance. I would not load for any "gas gun" without using a cartridge case gauge. Best practice is to run each case through the case gauge at the time the loaded round comes off the press. At the very least, spot check every few rounds in a given lot of reloads (for this to be acceptable, one must keep all brass in lots that have been fired the same number of times).
To illustrate the importance of the above statement, had a friend who is a very experienced highpower competitor and reloader who wrecked his match grade M1 Garand using the same handloads he has used for years when the rifle slam fired out of battery. Bent his op-rod, blew extractor/ejector out of bolt and rounded the receiver locking lug recess about 1/8" showing the bolt lug had barely entered the recess and was not in battery at the time the slamfire occured. He received a cut on the forehead and had a little problem with his trigger control for a while afterwards. Fortunately, the rifle, and his shooting ability have since been repaired.
Upon examination of the remaining lot of ammo, we found that the rounds did not have sufficient headspace (clearance) in his snug chamber. He had loaded this batch of ammo using the same die setting as always and other lots measured OK. The problem stemmed from the fact that this particular lot of brass had been fired quite a few times and had work hardened and his dies had been set to give proper headspace with once fired brass. The harder brass springs back more than softer brass after sizing.
Hopefully, this information may help someone else avoid this pitfall.
Regards,
hps
