Never, ever, ever neck size for a Garand/M1a action.
This mechanism has a free floating firing pin that is retracted only at cam down. The receiver bridge is an ineffective barrier at blocking the firing pin from touching the primer during feed as its primary function was firing pin retraction during extraction.
This is a receiver bridge from a M1 carbine, functionally it is identical to that of a M1a.
This is the Garand/M1a receiver bridge. You can see the slot when the firing pin clears the bridge.
In terms of bolt lug rotation, that pencil mark on the bolt lug is when, during cam down, that firing pin is able to clear the bridge.
If you have an overly long case, you are requiring the bolt to stop, to crunch fit the case to the chamber. In these mechanisms this is highly dangerous.
This picture, the firing pin tang is just touching the receiver bridge and the firing pin is fully forward.
At this point, when the bolt stops, that free floating firing pin is wacking the heck out of the primer, and your lugs are not engaged. If you have a sensitive primer and the bolt lugs are out of battery, this is what happens:
The unfortunate whose receiver heel blew off in his face said that his cases were inadequately sized. He was using CCI #34 primers, the proper primer to use in these mechanisms, he positively stated that he had reamed the primer pockets to depth and all primers were examined, and they were below the case head. He also said he put a round in the chamber and dropped the bolt. He must have had a sensitive primer and the lugs were not in engagement when it went off. If he had properly sized his cases his chances of having an inbattery slamfire would have greatly increased.
Never, ever drop a round in the chamber and trip the bolt release. In this mechanism always feed from the magazine. When a round is fed from the magazine the friction between magazine and round slows the bolt and that reduces the kinetic energy of the free floating firing pin. This will reduce the chances of a slamfire.
Always full length resize for these actions. Always size your cases with a small base die. Always set up your dies with a Wilson type case gage, and always use the least sensitive primers. The best primers for this mechanism are CCI #34’s and the Tula7.62 as these are said to be “mil spec” primers. Never , ever use sensitive primers in this mechanism. The most sensitive primers on the market and the most slamfiring primer in Garands/M1a are Federals. Avoid other sensitive primers such as "benchrest" primers. They are not appropriate for this mechanism.