Could be the primers seated too deep. OR, the gun not fully locked up, and the firing pin not going all the way forward, as there is (supposed) to be a stop to keep it from going forward and firing without the bolt being locked up. A friend, who's the Garand Guru, built the match rifles for the Army Marksmanship Unit when they still used them, advises against loading for the Garand, due to the floating firing pin, they can fire, when the bolt suddenly stops, do to too long a case, primer not seated deep enough, etc. He has seen a lot of them blown up. Also, the Garand should only be used with a couple of powders, 4895, which was developed for it, Varget, maybe R15, as the pressure curve has to be right to keep from damaging the op rod, etc.
