These are the two types of firing pins used by Springfield during WWII. The round firing pin on the bottom
was the original design and the firing pin used through about serial number 500,000. The design was changed to the half
round type at this point. Rifles had the round firing pins changed out during rebuilds. The round pin was bad to
rust in place or be "jammed" up by dust, dirt, mud or muck. The half round pin had less friction and surface area
touching the inside of the bolt. In later times with the development of more sensitive primers, slam fires could also
result from the increase inertia of the round pin.