If the rifle is mechanically correct, primer sensitivity is the primary cause of slamfires. While broken rifles with busted or frozen firing pins will cause slamfires, these are not subtle, and are easily detected. Murray’s Guns has a video of a mechanically defective SKS. It slamfires each time the bolt goes forward.
http://www.youtube.com/watch?v=xj3QtnUWCwQ. The most puzzling slamfires and the ones that really shake you are the ones due to primer sensitivity.
Primers vary in sensitivity by brand and within brand. Primer cake is a composition of a number of chemicals, which can be varied to make the primer more or less sensitive. People who have been at primer manufacturers told me that primer cake is mixed by hand with paddles. Anything mixed that way is going to have a bit of non uniformity and even if all the chemicals are of correct strength, I don’t doubt that some of the mix is going to be a little less sensitive and some of it a little more.
Anything with a free floating firing pin is susceptible to a slamfire given that the kinetic energy of the firing pin is enough to set off the primer.
I think Mr Oldham’s M1911 drop testing is interesting in this regard. Mr Oldham varied firing pin weight and drop height, using the same primers. You can see that not all primers went off at the same height, even though some did, showing that even within the same brand, primer sensitivity varies.
Drake Oldham M1911 Drop testing
The original testing used a 9mm steel firing pin and a 9mm titanium firing pin. The firing pin hole was then reamed for a .45 sized pin and the tests were repeated with .45 sized steel and titanium firing pins. All of the firing pins were weighed prior to testing. A Wolff XP firing pin return spring was used for all of the testing. All of the cases used for testing used Winchester large pistol primers. The frame and slide were donated by Gary Smith at Caspian. The pistol was built using techniques learned from Larry Vickers and Bruce Gray. The pistol was tied to a section of 550 cord, looped over a pulley, and dropped onto common floor materials. The magazine was loaded with 8 dummy rounds to bring the pistol up to proper weight. Four floor types were selected. Concrete, Pergo, 5/8 plywood, and shag type carpeting. The thumb safety was left OFF as preliminary testing with the safety ON indicated that damage to the thumb safety, slide, and plunger tube would occur with only a few drops. The hammer frequently dropped to the half cock notch during testing.
9mm STI titanium pin 2.17 grams
9 mm Caspian steel pin 4.45 grams
45 STI titanium pin 2.36 grams
45 Colt steel pin 4.30 grams.
I was amazed at how easily a Series 70 1911 could be drop fired. Steel firing pins and concrete are a bad combination. 9mm sized pins and titanium construction will add several feet to your safe drop distance. I will be running Wolff XP springs and a Ti pin in all of my Series 70 type 1911’s.
I have attached an Excel spread sheet with the results. You will notice a lot of “Did Not Drop” entries. I saw no reason to drop test a particular combination of firing pin and flooring if it was not firing at higher distances or on harder flooring. I did several drops at various distances to get an idea of safe drop distances. This was to account for hard or sensitive primers. Each primed case was dropped only once. Just in case you were wondering, the pistol sustained significant damage. The muzzle is distorted from being dropped. I had to turn down the outside diameter of the barrel three times just to keep the slide from locking up. The muzzle, magwell, and grip safety have some serious blending in their future. Nothing sounds worse than a 1911 hitting the concrete from 10 feet!
His excel spread sheet can be found:
http://www.drakesgunworks.com/Drop_Testing.html
High primers are a red herring as it is in fact very difficult to get a high primer to go off. The anvil has to be firmly seated on something and the primer cake has to be pushed into the anvil. This can happen if there are shallow primer pockets and one guy got his ammunition to slamfire by not removing the brass donut that results after swaging the crimp from a military round. He swaged his cases and left the donuts in the bottom of the pocket and seated primers on top of that.
Today many people still follow the lead of the American Rifleman technical staff who did not acknowledge primer sensitivity as a cause of slamfires. I went through every American Rifleman magazine from the late 50's up to 2000 and primer sensitivity as a cause is only touched on briefly in Wayne Faatz’s article “The Mysterious Slamfire”.
http://www.scribd.com/doc/2649554/The-Mysterious-Slamfire and the whole concept repudiated years later by a lead staff writer and retired Frankfort Arsenal Employee. In print he wrote that the idea that the Garands/M14’s might have a design defect “preposterous” . The only allowed causes of slamfires were high primers and worn out receiver bridges.
This is puzzling as their most knowledgeable Staff writers had worked for Army Ordnance, at least one of them worked at Frankfort Arsenal for decades and I have copies of letters William Davis (a NRA writer) sent out to all American Ammunition manufacturers concerning his study of primer sensitivity and the AR15. If you don’t remember, early AR15’s slamfired which resulted in the Army specifying a less sensitive primer and a lighter firing pin. These ex Frankfort Arsenal employees also went through development of the lead styphnate primer, something that was tested over years concerning issues you never thought of, including weapon system character tics and primer sensitivities. I have a couple of these reports. The lead Staff Expert at the NRA actually ran the Smalls Arms Test Division at Aberdeen after WW2 when all prototype rifles were tested (and there were lots of them) leading to the selection of the M14.
And yet with all this background in ammunition manufacture, investigations into primer initiated slamfires, they never mentioned primer sensitivity as a factor in slamfires in the Garand or M14.
I believe this was a misdirection away from those "preposterous" defects inherent in Garands/M1a’s and the AR15, all of which have free floating firing pins and all have been known to slamfire. Unfortunately the Garand/M1a will slamfire out of battery.
Civilians did not know about AR15 slamfires at the time, the mass importation of military surplus rifles with heavy free floating firing pins had not occurred. Such rifles include the Tokarev, AK47, SKS’s, MAS49/56 and you can easily find slamfire reports on them. And of course there was no internet for shooters to compare notes. The American Rifleman staff controlled the in print theoretical discussion about slamfires by only publishing dope bag articles and accounts that supported their contentions that only misconduct by shooters cause slamfires. Ed Harris was a NRA technical writer in the 80's, here in this post, it is all about your poor reloads and your out of spec rifle. Barely a nod to primer sensitivity, all this shows the mind set of the American Rifleman staff of the period.
http://www.thehighroad.org/showpost.php?p=6534244&postcount=7
A lot of people have had slamfires in Garands/M1a's and have been hurt following the advice of the NRA Technical Staff.
Slamfires can and have happened with all rifle primers, and why is a long discussion, lets just say primers are not as predictable as people want them to be. People who use the most sensitive primers on the market are just increasing their chance of a slamfire.
I recommend in all mechanisms with free floating primers to slow the bolt speed down by feeding from the magazine and to use the least sensitive primers you can find.
For Garands/M1a’s it is critical to small base size cases and by the use of Wilson Type case gages, ensure that the case is sized below chamber headspace. Set the shoulder back about 0.003”.
Even though this guy used CCI #34 primers, he had a tight case. When a Garand/M1a bolt has to crunch fit a tight or long case the bolt lugs are not engaged and the free floating firing pin is unrestrained and will hit the primer at the highest velocity of bolt travel. This guy dropped a round in the chamber and released the bolt and blew the back end of his receiver off.
It is possible that had the case dropped all the way in, the slamfire would have been in battery, though given the design of these rifles, that will never be a 100% certaintly.
) But I digress...
