I think RCBS is getting the whole wrong impression of slam fires.
There are a half dozen or more causes of slam fire in semi auto rifles.
Conventional wisdom is that most occur from improperly seated primers.
But there are indeed a half dozen other causes, like frozen firing pins, etc.
I do not believe the statement "Slam fires are least likely, or won't occur, with small base dies"
is a correct statement, especially when you consider the other dozen or so causes.
Yes there are more causes to slamfires than primer sensitivity. I discount mechanical problems, not because they don’t happen, but because they are not subtle.
If you have a firing pin frozen forward that defect is going to be very obvious on the first round you chamber. Jack O’Connor had a busted firing pin shaft on his 03 Springfield. If you are not familiar with the 03, the firing pin is pushed forward through the firing pin hole when the shaft breaks. The cartridge ignited as he pushed the bolt forward. The bolt blew out the receiver almost tearing his thumb off. And that was with a bolt rifle.
Back in the day when M1a’s ruled the line, and before decent AR15 trigger mechanisms, you always saw a trigger mechanism follow. Someone had tuned their trigger sear surfaces to a minimum, the sear wore with use and the rifle would double during rapid fire. Boom, boom. We usually gave the guy an alibi, the rifle would always follow before the next ten round string was finished, and after that we kicked the rifle off the line.
Mechanical problems are about as subtle as a head on collision with a Mack Truck.
Assuming your firearm is mechanically correct, reducing the chance of a slamfire is a process. With mechanisms with free floating firing pins the probability of a slamfire is never zero. As long as primer sensitivity varies, and as long as a firing pin is tapping on a primer, there is a chance that sufficient energy exists to ignite that primer.
I believe the process starts with full length sizing with small base dies. And sizing the case below the dimensions of the chamber. For guns with free floating firing pins, like Garands/M1a’s/ M1 Carbines, you want absolutely no delay to bolt turndown. If you are going to have a slamfire you want it in battery.
The next part of the process is to ream primer pockets to depth. Wayne Fatz was able to show that high primers can ignite after impact with the bolt face. Reaming primer pockets to depth greatly reduces the chance of a high primer.
Seating primers by hand and verifying that all primers are below the case head is the next step in the process.
An important part of the process is to use the least sensitive primers you can buy. There are lots of reports of slamfires in AK’s, AR’s, Garands, Carbines, FAL’s, with sensitive primers. Again that free floating firing pin is just tapping the heck out of the primer as that bolt goes forward. Less sensitive primers are less likely to ignite.
Still if you do all this, you can still have a slamfire. You have reduced that risk, you have reduced the chances of an out of battery slamfire, but given the number of reports of slamfires with factory ammunition, service rifle ammunition, only proves that XXXX happens.
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.
This weapon slamfired in battery. No one was hurt. But it was uncontrolled.
http://www.youtube.com/watch?v=bu8Dwj7Ey8k
