A gun is any of several types of object that propels another object or projectile. The projectile is generally fire through a hollow tube known as the gun's barrel. The barrel's diameter, or caliber, determines the size of projectile used, which is usually designated in fractions of an inch or in millimeters. Differing from the musket, most modern guns are rifled, with a series of grooves spiraling along the barrel; exceptions include smoothbores on tanks, AFVs and some artillery.