Diameter of the bore of a gun. In rifled gun bores, the caliber is obtained by measuring between opposite lands. A caliber 0.45 revolver has a barrel with a land diameter 45/100 of an inch.

It is diameter of a projectile.

It is a unit of measure used to express the length of the bore of a weapon. The number of calibers is determined by dividing the length of the bore of the weapon, from the breech face of the tube to the muzzle, by the diameter of its bore.

A gun tube whose bore is 40feet (480 inches) long and 12inches in diameter is said to be 40 calibres long.