firing pin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The part of the bolt or breech of a firearm that strikes the primer and detonates the charge of a projectile.

from The Century Dictionary.

• noun That part of the breech-mechanism of a small arm or cannon which explodes the primer of the cartridge.

from the GNU version of the Collaborative International Dictionary of English.

• In the breech mechanism of a firearm, the pin which strikes the head of the cartridge and explodes it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun firearms the part of the firing mechanism which strikes the cartridge's primer to ignite the gunpowder.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun striker that ignites the charge by striking the primer