If you're meaning is other than a
bullet as in projectile fired from a firearm, forgive me. Basically, a "shell" made of brass is filled with gunpowder. at one end is fitted a firing cap or "primer" that resemble a metal encased cap from a cap gun. At the other end is a "bullet" fitted very tightly into the end of the case. When placed in an appropriate firearm, the bullet end fits into the barrel of the gun. When ready, the trigger is pulled and a pin strikes the firing primer which, in turn, explodes the gunpowder, which is turn releases tremendous energy to the base of the bullet propelling it down the barrel of the firearm towards its target. Examples seen here:
http://www.programmers-progress.com/images/bul lets.jpg
and here:
http://www.fotosearch.com/DGV035/200242590-001 / ...