Has it been fired in a military spec 7.62 rifle? There are meant to be differences, as this bit of random internet chatter says:

Indeed, cheaper commercial ammo canfailat the 1.638 headspace (e.g., UMC) in an M14/M1 Garand. Many military gas guns (e.g., M14 Rifles & M60 Machine guns) runwildlylong headspace by commercial (SAAMI) standards (U.S. Militaryfieldreject limit for the M60 & M14 is 1.6455, nearly 16 thousandths beyond commercial (SAAMI) GO, & nearly 8 thousandthsbeyondcommercial (SAAMI)fieldreject limit!).