Handguns with reciprocating actions such as semi-automatic pistols, will often use a firing pin that is too short to project when depressed flush by the hammer. This type of firing pin, called an "inertial firing pin", must be struck by a full fall of the hammer to provide enough forward momentum to reach and strike the primer. If the hammer is down, resting on the firing pin, it is very unlikely that an external blow to the rear will provide enough kinetic energy to the firing pin to detonate the primer. Most variants of the M1911 pistol use this type of firing pin.