percussion cap

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

• noun A thin metal cap containing an explosive substance, such as fulminate of mercury, that explodes on being struck.

from The Century Dictionary.

• noun A small copper cap or cup containing fulminating powder, used in a percussion-lock to explode gunpowder.

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

• noun a metal cap found on muzzleloading firearms that contains chemical substances such as fulminate that explode upon impact

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

• noun a detonator that explodes when struck