[Steve Smith]

The M1, M14, AR15s have floating firing pins, meaning they don't have a spring to hold them back when the bolt closes. Because of this, if you have a filthy chamber and the cartridge stops short, or a too-tight headspace (same thing as filthy chamber), or a high primer, or a bolt going too fast, the firing pin can hit the primer and ignite the cartridge. If you charge the rifle, and then extract the loaded round, you'll see that the firing pin has already hit the primer. Completely normal.


Again, I think you doubled the rifle with a milked trigger pull (you didn't follow through with your finger), and under recoil the sear reset and then in the "negative" or "reverse" recoil the sear tripped again because your finger had not moved, firing the rifle. I don't know of many that have not done this when learning the ropes.


BTW, it is imperative that you single load an M1 or M14 from the magazine (the M1 has a fixed internal magazine) rather than plopping the cartridge in the chamber or in top of the follower and closing the bolt. With the M1, insert the en bloc clip, then put a round or two under the clip lips and on the follower. This will slow the bolt slightly and reduce the chance of slamfire, which happens most often when single loading.