What does 50 caliber mean?

Answer 1

Caliber refers to the diameter of the bullet. A 50 caliber bullet is .50 of an inch (half an inch) in diameter. It's also used to refer to a gun that shoots a 50 caliber bullet.

The problem with the word "caliber" is that it has two different and confusing meanings. In small arms, "caliber" usually refers to the diameter of the bullet, NOT the casing, and is measured in inches (i.e. .50 caliber is 50/100ths of an inch, or a half-inch). [Note: your original question should be ".50" caliber, not "50" caliber.] The biggest problem with this usage is that it's not exact - the most common .50 caliber bullet (the .50 BMG of the Browning .50-caliber Heavy Machinegun) is actually .510 inches at the widest point.

The other use of the word caliber is the ratio of the barrel bore (inside) diameter to its length. For instance, the huge 16" guns on the US's Iowa-class battleships are properly classified as 16"/50-caliber guns, which means that the barrel length is 15 times the bore length; thus, the 16/50 is 800 inches long (66 feet, 8 inches). While this usage of caliber is most common for cannon (and not small arms), it nonetheless can be found in discussions of rifles.