As Hans said. Springfield offers a pamphlet that addresses this. The firing pin does not have a spring to cushion the blow. When the bolt goes into battery, it's possible that the firing pin can hit the primer without the hammer striking the pin. Many times it's a light dimpling on the primer face. CCI offers a #34 Mil-Spec primer which has a thicker cup. Magnum primers have thicker cups as well if you are rolling your own. Surplus 7.62 ammunition such as from Lake City Arsenal, as well some ammunition from companies such as Winchester, have Mil-Spec primers.
Incidentally, you can get slamfire from a Garand or the AR platform as well. It's attributable to the primers used.
Regards,
Bill