While slam fires are not to be taken lightly and good safe gun handling practices and good safe reloading practices should always be followed, slam fires suffer a bit from the "Chicken Little syndrome".
Agree and disagree.
Based on web reports, Garands have the most slamfires of any surplus military rifle. If there were an equivalent amount of MAS semi autos that would probably be the most slamfiring rifle, based on the descriptions on the weight of the firing pin, but there are more Garands in the hands of civilians than at any time and that mechanism has the most slamfire reports of any mechanism.
The basic problem with Garands/M1a’s/M1 carbines/Mini 14’s is that the firing pin is free floating and is able to contact the primer throughout the entire cycle. There are those that claim the receiver bridge provides protection, something that is debatable, however regardless of claimed functionality, the receiver bridge is not a positive barrier and is easily defeated.
Slamfires are a matter of kinetic energy and primer sensitivity. Primers vary in sensitivity within the lot and depending on primer composition chemistry.
The Garand went through several firing pin changes to reduce kinetic impact energy. The rare round firing pin lasted until the early 40’s and was replaced with the lighter scalloped firing pin. Still enough slamfires occurred with that firing pin that the Italians installed a spring around the firing pin in their Garand based BM rifles.
The M14/M1a has a shorter firing pin, and thus by weight alone, a lighter firing pin, but even so, there are accounts of M14/M1a slamfires, in battery and out, with military primers.
I do not agree using extra power recoil springs in these mechanisms as there are reports of slamfires after a spring change out. Extra power springs may also mess up the timing. Stick with standard recoil springs.
I believe the Government adjusted the firing pin weight and primer composition sensitivity (as much as they could without causing misfires in the other 30-06 weapons around) so the probability of a Garand slamfire would be in the 1:50 or 1:60 million range. This is based on a William Davis recommendation for primer sensitivity and slamfire probability in the Ar15’s.
People win State Lotteries every month with 1:50 or 1:60 million chances, and people die of snakebite (History Channel said about 5 people in the US every year) so the chances of dying of snakebite are about the same. And if you have ever noticed, most people freak over snakes, killing all snakes, regardless of whether they are poisonous or not. Poor snakes.
Slamfire probabilities increase with sensitive primers, Wiliam Davis showed with sensitive primers the slamfire probability in a AR15 increased to 1:9000. There is absolutely no data on what slamfire probabilities with Garands/M1a’s, etc, with any primer, but given a choice of primers, I want to use the 1:50 million primer, not the 1:9000 primer.
The reason is the seriousness of the consequences. While the probability is low the outcome of the event can be very bad. If it is an out of battery slamfire then you will get brass particles all over the place, wood particles, in your hands, in your eyes if you are not wearing glasses, and the destruction of your receiver is a likely outcome. If it is in battery, bad things can still happen. A retired bud of mine remembers when a Soldier loaded a Garand with live ammunition in a M113, before the hatch went down, the Garand discharged, bullet going through the arm of one Soldier, ricocheting off the hatch and back into another. I can’t prove it was a slamfire but an unexpected discharge of a 30-06/308 can have all sorts of unattended consequences. There are also a number of reports of shooters whose palm was cut open by the operating rod handle, because the rifle slamfired before they got their hand out of the way.
Still, regardless of what you do, proper primers, proper sizing, there is a slamfire chance with these weapons. You want to stack the odds so that it is in battery and that nothing downrange gets hit when you chamber a round.