Buried deep in our culture is the idea that high primers cause slamfires. This is part of a coverup, created by the ex Ordnance Officers who worked for the NRA. In the early 60’s Garands started getting into the hands of civilians and they started having slamfires. Rifles were blown up no doubt. Since the M14 had the exact same bolt mechanism as a Garand (civilian could not own M14’s) and Colt had an incredible anti M14 smear campaign going on, it seems obvious to me why the still Army faithful NRA tech writers did not mention that the Garand/M14 mechanism would slamfire, and slamfire out of battery, with sensitive primers.
So they created a story that only “high primers” and worn out receiver bridges (specific to Garands/M14’s) were causes for slamfires. This passed the buck to shooters as the Army could not be responsible for old worn out guns and sloppy ammunition. They never ever, ever mentioned primer sensitivity. I believe that is because firing pin energy is intrinsic to a design and for the Garand/M14 the firing pin is only retracted on cam down. Garands and M1a’s will slamfire out of battery (and in battery) with sensitive primers. The AR15 design extends the firing pin on cam down which is why you never hear of out of battery slamfires in AR’s.
Primer sensitivity is in fact the primary cause for slamfires in mechanisms with free floating firing pins.
As for high primers, it is exceedingly difficult to get a high primer to go off.
Someone here has a link to a CCI page which states that high primers are the most frequent cause of misfires. The anvil has to be set firmly on something and it has to be against the pellet.
Here is an example of a high primer that did not go bang.
http://www.reviewsofthings.com/guns/9mm_luger_primer_failure_misfire_dud.html
Wayne Faatz wrote an article about slamfires in Garands in the American Rifleman. It was because he had one with Federal primers, the most sensitive primer on the market. He ran an experiment. He seated rifle primers high and they would not slamfire. Instead the bolt face seated them flat. He then tried pistol primers and they would not slamfire and the bolt face seated them flat. He finally inserted a flattened anvil in the pocket, hard seated a pistol primer, which was now above the case head, and that went off when the bolt was dropped.
So it is possible for a high primer to go off if the anvil is sitting on something hard, like extra primer anvils, washers, coins, spring coils, shellfish, lava flows, tombstones, etc. Undoubtedly one will go off if the pocket is really shallow. But then why did it not go off when it was a factory round?
The Army was having all sorts of issues, slamfires, primer sensitivity, etc, with the early M16’s. Early 223 ammunition used sensitive commercial primers. The quote from the primer sensitivity section was
“Contrary to the requirements for 7.62 mm ammunition, which specifies that the FA34 primer is mandatory, no attempt has been made to standardize on type of primer for 5.56 mm ammunition”.
You can find this at
http://www.dtic.mil/dtic/index.html.
This is on page 4-69 of the "Report of the M16 Review Panel Appendix 4 Appendix 4 Ammunition Development Program"
To reduce slamfires in AR’s the military required the use of a less sensitive primer, the #41 primer. The Army did three things: they reduced the firing pin weight, required a thicker cup in the primer and a less sensitive primer mix.
I stole this picture from AR15.com. The early heavy firing pin is on top.
I had one slamfire in a NM AR15 with the new brass WSR. I dropped a round in the chamber, hit the bolt release, and the rifle discharged. My scorer, when he got up to shoot standing, the same thing happened to him, he was shooting Federal match in his AR. Since then I found out he had another slamfire with Federal match and stopped using those primers. We both modified our standing loading routine, we now ride the bolt down half way with the charging handle, before letting go.
This might be of interest
Tavor 21 Slamfire video on youtube.
Notice how many rounds the guy fires. If he had a mechanical problem he would have 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. Winchester redesigned their primers in 1999 to make them more sensitive.
http://www.youtube.com/watch?v=bu8Dwj7Ey8k
Factory built rifle or parts gun?? 
