I have fired many thousands of rounds through various gas guns using regular primers and have never had a slamfire in one of my rifles. Most of my experience has been with M1/M14’s, only recently with the AR’s and that to a much more limited degree. Having said that, I do use CCI #34 mil spec primers when loading for the M1/M14.
http://www.cci-ammunition.com/products/primers/primers.aspx?id=30
Thankfully, slamfires are a relatively rare occurrence in any gas gun, but they have been documented, especially in M1’s & M14’s.
A slamfire in either the M1 or M14 can have disastrous results if it occurs before the bolt is in battery. The bolt design of the AR would seem to make firing out of battery nearly impossible since the firing pin is not long enough to reach the primer until bolt locks.
Lack of care in following careful reloading practices or a bit of debris on bolt face, or perhaps a broken firing pin, combined with a soft primer could cause a slamfire, even in an AR.
While the following concerns the M1, hopefully it will illustrate what can happen should a rifle slamfire while not in battery. I have personal knowledge of several other such events that I did not actually witness. One was an M14 using military issue ammo in which the shooter received some serious injuries, so the hard primer is not absolute insurance against a slamfire.
A friend who is a very experienced highpower competitor and reloader wrecked his match grade M1 Garand using the same handloads he had 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 (thankfully) had barely entered the recess but was not fully in battery at the time the slamfire occurred. He received a cut on the forehead and had somewhat of a problem with his trigger control for a while thereafter. Fortunately, the rifle, and his shooting ability have since been restored.
Upon examination of the remaining lot of ammo, we found that the rounds did not have sufficient headspace (clearance) in his snug, match chamber. He had loaded this batch of ammo using the same (full length) die setting as always. Remember, all previous lots had measured OK. The problem stemmed from the fact that this particular lot of brass had been fired at least 8 times and had work hardened. His dies had been set to give proper headspace with once fired brass and he failed to check headspace on this lot after loading. The harder brass springs back more than softer brass after sizing which resulted in oversized (for his chamber)rounds.
Ammo is probably the number one cause of slam fires in the Garand & M14. 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! Other issues that can cause a slam fire are dirty chamber, broken firing pin or a pin that is no longer free floating for whatever reason.
Soft primers can contribute to a slam fire. The CCI #34 & #41 military primers have a hard cup to duplicate GI ammo, most of which is loaded with a hard primer. Handloaders must be aware of two other factors that can result in 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 your rifle's chamber giving proper headspace clearance. I would not load for any "gas gun" without using a cartridge case headspace gauge. Best practice is to run each case through the case gauge at the time the loaded round comes off the press after determining the actual headspace required for your rifle; remember, all rifle chambers are not created equally. 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).
Hopefully, this information may help someone else avoid this pitfall.
Regards,
hm
