If you look at the book, the Black Rifle, early M16's were slamfiring out of battery when loaded single shot. The Army lightened the firing pin to reduce the impact energy.
Primers on commercial ammunition are extremely sensitive. I think this is due to two things, first that gunwriters promote this as a desirable characteristic and because of the loud screams of the coil cutters. There are these vocal people who have ancient mainsprings or just cut coils off their mainsprings and have ignition problems. Of course it is the primer’s fault, not theirs, and they let the world know about it.
Primer manufacturers don’t like negative press, so they have made their primers more sensitive. This is what Winchester did in 1999. They changed their product line to make them more sensitive “to off center hits.” Unfortunately, the brass WSR primer now pierces at loads than never bothered the old nickel plated WSR. Others have reported slamfires with WSR.
http://www.thehighroad.org/showthread.php?p=4627417
Slamfires are a risk with any system with a free floating firing pin. I had one in a AR, and seen another one. Mine came during standing slow fire where you are required to load one round at a time. I dropped a round in the chamber and hit the bolt release. My rifle slamfired, taking a divot out of the ground, about ten feet away. I got some real scared looks by the competitors around me. I was using the new brass WSR. My scorer, a HM with several President’s 100 patches, laughed at me during the relay change. The capacious range gods decided to teach him humility and his AR slamfired during his record period. He was using Federal match primers.
Since then I have gone to CCI#41 primers for the primer sensitivity and because of the number of firing pins I had to replace when the WSR pierced. CCI has a military primer product line, their primers are less sensitive, and they shoot just fine in my AR’s.
The NRA had to change their rule book due to primer sensitivity. It used to be that during standing you could load your rifle on the stool. I used to see guys resting the muzzle of their AR’s on their shooting stool, drop a round in, and hit the bolt release. I highly suspect the rule was changed because someone put a bullet in their stool when their AR slamfired.
If there is a good thing about the AR, unlike the Garand or the M1a which will slamfire out of battery, I have never seen an out of battery slamfire with the AR.
Still, these things will slamfire. Therefore you must always make sure of what is downrange when you load. Take a look at the slamfire of this Tavor 21 on you tube. It is not a AR, but apparently these will also slamfire.
Notice how many rounds the guy fires. If he had a mechanical problem he would have had recurring slamfires, but he did not. When you see the slamfire, notice that the finger is not on the trigger. He was running Federal American Eagle (federal primers) and Winchester ammo.
http://www.youtube.com/watch?v=_cv7BI3wGWA
