Inertial Firing Pin

Noun (ĭ-nûr′shə)

A firing pin that is too short to contact the cartridge's primer when the hammer is resting on it and can only hit the primer when driven forward under the momentum of the hammer blow. The system used in the Colt 1911 is an example of an inertial firing pin.

1911 showing internal firing pin
1911 showing internal firing pin