A firing pin is a movable pin in a firearm that strikes the primer of a cartridge to set off the charge. In the case of the AR-15, it is located in the bolt carrier group and can be easily exchanged in the event that it is damaged or ceases to function properly.

In the AR-15 bolt carrier group pictured here, the firing pin is the silver-colored item in the middle of the picture.

In the AR-15 bolt carrier group pictured here, the firing pin is the silver-colored item in the middle of the picture.