Because the bolt moves much faster without the drag of stripping the round from the mag. When it slams shut, the free-floating firing pin can ignite the primer.
It the same process that causes the small primer indent on unfired rounds loaded from the mag, but in that case the extra speed causes the round to be ignited. It won't happen every time, but it can happen.
That's why I keep expecting Jamie and Adam to end up with a Garand facial on Mythbusters.
Sage advice...... When you explain it like that it makes perfect sense!


















































