What is the meaning of nipple of firearms?

Answer 1

Before modern firearms that use cartridge ammunition were made, there were muzzle loading firearms. To fire these, you pour loose gunpowder down the barrel, ram a lead ball down the barrel, and then place a percussion cap (small metal shell containing a pinch of explosives) on the gun's nipple. This is a hollow metal stud at the rear of the barrel. The hammer strikes the cap, crushing it against the nipple. It explodes, and sends a jet of fire through the hollow nipple, igniting the gunpowder in the barrel.