In the U.S., caliber is generally given in inches, with 1.0 equaling one inch. A semiautomatic weapon fires one shot every time the trigger is pulled. An automatic weapon fires continually until the trigger is released. Both are handguns. A revolver contains a revolving cylinder in which bullets are loaded.
Revolvers usually hold six shots.
The ATF defines a pistol as any handgun that does not contain its ammunition in a revolving cylinder. Most pistols have a removable magazine into which bullets are loaded. The magazine is then inserted into the hollow handgrip of the gun. A dropfire is an accidental discharge of a firearm caused by a gun hitting the ground or other surface. For a pistol, the shock from impact causes the gun's spring loaded striker mechanism to jump loose from its safety and snap forward, striking a bullet in the gun's chamber and firing the round.