I am getting set-up to reload again after a long time away, (if they ever ship the stuff). I'm getting things I did not have before. I understand about slam fires and free floating firing pins as 98% of my inventory is that way. So that is why I asked as the finer points need refreshing in my mind. I'm only going to do 7.62x51 at first using NATO brass. Diamond K washed cases arrived today, which reminded me I need a swager, so I think I will add a gauge to my order also.
If you are reloading for a Garand or M1a, use the least sensitive primer you can. I recommend the CCI #34's because CCI says these are their military product line, which means a less sensitive primer than the commercial product line. Do not use Federals as they the most slamfiring primer on the market. When I had my slamfires, with Federals, I asked around and the one common theme to those competitive shooters who had had out of battery slamfires, was Federal primers. Federal primers are great in bolt guns but they are too sensitive for the Garand mechanism.
The thing is, the Garand/M14/M1 carbine/Mini 14 mechanism has, and will slamfire out of battery with military and factory ammunition. Every so often an extra sensitive primer comes out of the batch and in feeding, the free floating firing pin rebounds off the primer, and it ignites.
Since you cannot visually inspect primer sensitivity, the only real primer control you have is buying primers which are on, the average, less sensitive. That reduces the chances of having an extra sensitive primer in a round and reduces the risk of a slamfire to an acceptable level. But it is always there.
Always wear your shooting glasses!!
Sizing the case to less than chamber length and width, that is making the case smaller than the chamber, reduces the chances of an out of battery slamfire, because the bolt will not have to crunch fit the case to the chamber. As I said before, when the bolt has to stop to crunch fit a case, that firing pin rebounds off the primer, and that may cause the primer to ignite. If you are using once fired military brass you are better off using the Sheridan gages and a small base die. Standard sizing dies will not reduce big swollen cases enough to drop in a Sheridan case gauge. Sometimes, small base dies are not enough to reduce a big swollen case to drop in a Sheridan case gage. That is when you need a Roll Sizer.
Rollsizer.com
Case pro
Take your pick, almost $1000 apiece. But this level of mechanical and hydraulic compression is what is needed if a small base die cannot reduce once fired brass to fit in the chamber without resistance. Be aware, the very act of seating a bullet bulges a case a tiny bit. It shows up in the gauge.
If your brass is still too large after sizing in a small base die, it is better to take that stuff and use it in a bolt gun, and go buy new, never been fired, brass, and use that in your Garand/M1a/M1 Carbine/Mini 14.
Always wear your shooting glasses and always load from the magazine. The mere act of of loading from the magazine slows the bolt down a bit. Controlling slamfires in mechanisms with free floating firing pins is all about primer sensitivity and the kinetic energy of the firing pin.
I even sent a Garand bolt off to Roland Beaver and he drilled and installed a firing pin spring.