The other day I was shooting 168 grain Hornady Match ammo out of my M-1. I had one round with a pierced and flattened primer. The round grouped with the others and was not a flyer. My M-1 was clean and properly lubricated. The firing pin moves freely in the bolt without sticking. Could this be the result of a defective primer? A round with more powder? Possibly a temporary blockage of the barrels gas port? Subsequent rounds had normal primer indentations. Any ideas?