A decade ago people were claiming that AR15's never slamfired. Now there are plenty of reports and Vietnam era material finally being declassified. Garand slamfires were getting to the limit of living memory when all these CMP Garands arrived and enough are being shot that it is not difficult to find reports of slamfires in the things.
One thing that totally muddied the water was an active disinformation campaign conducted by the NRA. When civilians started getting their mitts on NM Garands, they started having slamfires. In the early 60’s the NRA was a quasi Governmental organization, with a line of military directors going back to the 1870’s. As a happy little tape worm in the colon of the Army, the NRA received a lot of support and resources from the Army. The NRA, in collusion with the Army, disavowed any idea that the Garand had design issues that could cause slamfires. There are a series of articles in American Rifleman “proving” that only bad reloads or worn out Garands will slamfire. This blame shifting has gone deep into the core of the shooting community and is the origin of the myth that "only high primers" cause slamfires. One Dope Bag article actually claims “do you think the Army would ever put a dangerous rifle in it hands of its troops”.
Would any Vietnam era trooper who had his M16 jam in combat ever believe that the Army would not put a dangerous gun in the hands of its troops? You can read the coverup in the book “The Gun” by C.J. Chivers. Truly patriotism is the last refuge of scoundrels.
In 1968 Congress forced an purgative down the Army’s throat, the NRA tape worm and Civilian Markmanship were flushed clean out of the Army's system. Since then the NRA is somewhat less willing to fall on its sword for all things Army.
If you are interested in reading more about the early M16 slamfire issues go to
http://www.dtic.mil/dtic/ and type in “Report of the M16 Rifle Review Panel. Volume 5, Appendix 4. Ammunition Development Program”. Read the other reports. These documents were classified as part of the Army coverup of the problems with the M16 program. We are fortunate that they were retained long enough to be posted to the internet.
What you will find in that declassified report is a discussion of primer mixes. There is a common assumption that a primer is a primer is a primer. This is false.
I have found additional material in DTIC, someday I will copy, showing the percentages of materials in the primer mix.
Manufacturers can and do change the sensitivity of the primer mix, make the cup harder, change the anvil angle and depth, and probably more things, to make primers more or less sensitive.
One thing that is also not recognised is that primer sensitivity follows a bell curve.
The following section is from Mil-P-46610E:
3.2 Sensitivity.-The sensitivity shall fall within the limits specified as follows for each type of primer:
Code:
Primer Required Case Height in Inches
H +5S H -2S
Dwg . No. B10522621 7.62mm Match 15 3
Dwg . No. B10535489 7.62mm Match 16 3
Dwg , No. B10522621 7.62mm 15 3
Dwg . No. B8594094 7.62mm Match 16 3
Dwg. No. B10522621 7.62mn Grenade 15 3
Dwg . No. B10522621 7.62mm Blank 15 3
Dwg . No. B10535489 7.62mm Blank -Caliber 16 ½ 2½
Dwg . No. B10522621 .30 15 3
Dwg. No. B10535489 Caliber .30 15 2½
Dug. No. A5000131 Caliber .30 15 2½
Dug. No. B8595819 Caliber .30 Match 15 2½
Dwg, No. B10535489 Caliber .30 Match 15 2½
Dwg. No. C7645332 Caliber .30 Blank 15 2½
Dwg. No. B8594094 Caliber .30 Blank 15 2½
Dwg. No. B6200959 Caliber .30 Carbine 18 2½
Dwg. No. C11751131 Caliber .30 Carbine 18 2½
Dwg. No. B7645336 Caliber .45 16 2½
Dwg. No, A5001168 Caliber .45 16 2½
Dwg. No. B7645339 Caliber .50 15 2½
George E. Frost, in his book “Ammunition Making”, provides a good description and procedure for calculating the sensitivity numbers in a primer test. Of interest, the H – 2S (two standard deviations) means that 4 primers in 900 would be expected to fire, and H + 5S means 3 in 10, 000, 000 primers are expected to misfire. So it can be seen that even at the lowest drop, there is a significant probability of ignition. That is why in a gas gun it is important to use a “hard” primer, and to follow reloading practices which minimize the chance of a fat case binding in the chamber before the bolt is locked.
The Garand is well known for having in and out of battery slamfires. You want to use the least sensitive primer you can find in mechanisms with free floating firing pins.
On page 58 of the April 2011 Guns Magazine Mike Venturino reports having a slamfire in a K43 and SVT40 rifle with standard primers. Thankfully each slamfire was in battery.
I understand Russian primers are also very insensitive.
If you are a High Power Shooter you will find shooters who have had slamfires in Garands and M1a's. I met one guy who blew the back end of his receiver off with Federal primers. He had a Wilson match barrel installed, sized cases with a standard sizing die, and that receiver is now junk. Joe said "people ought to know how dangerous these things are to reload for". Doug had an out of battery slamfire in his 308 NM Garand, with Federal primers. Gulley blew his M1a stock apart with Federal primers when it fired out of battery. Gulley was a new shooter at the time and was copying a Marine Corp reloading technique. He had seen the USMC shooters place a round in their chambers and unlatch the magazine. Apparently they were concerned about scratching the bullet tip. With a round in the chamber they would then pull back on the operating handle and let the bolt go forward.
I would recommend that no one ever follow this practice. M1a's should always be fed from the magazine to reduce the forward velocity of the bolt and Garand shooters should use a SLED if firing single shot for the exact same reason.
You can also find reports of M1 Carbine slamfires. The mechanism is similiar, uses the receiver bridge to retract the firing pin. Since the firing pin only engages the bridge just at bolt turndown, if there is bolt bounce going on, the mechanism, just like a Garand, can slamfire in and out of battery.
). I have been extremely conscientious about ensuring that the primers are fully seated, something I consider essential in any semi (rifle or pistol)... 
