With all the talk about firearms in the news lately, as well as the items about delivery of artillery to Ukraine, the term “caliber” (or its Euro spelling “calibre”) is seen often in the press. Here is a brief guide to avoid confusion.
In small arms, the term is used to denote the diameter of a bullet, or the diameter of the bore of the barrel it is fired from. For example, a .45 caliber bullet is 0.45 inches in diameter. A 9mm bullet is 9 millimeters in diameter. The caliber may be expressed in either inches or metric units.
Since many bullets may have the same diameter as other bullets, but otherwise very different characteristics, sometimes the caliber reflects this difference, so that the wrong bullets are not placed in a gun where it fits, but which it was not designed for. For example the .38 Special bullet AND its brass casing is identical in dimension to the .357 Magnum round, and the former can be fired from a revolver chambered for the latter. However, the .357 carries a much hotter powder load, and the reverse is not necessarily true. A .357 round fired in a .38 chamber may develop too much pressure, causing an explosion, and possible injury to the shooter. The .380 caliber round, and the .38 Super Auto are semiautomatic pistol rounds and totally different from one another, and other “thirty eights”. Neither is designed for use in a revolver. There are many other examples of this, with enormous implications for safety and convenience. It is best to keep in mind that caliber in this sense is a verbal descriptor, not a ballistic measurement.
In artillery, the term caliber is used in a totally different way. For example, the naval guns on the ship I trained on in the 1960s were designated 5″/38, or five inch thirty eight calibers. This means the projectile was five inches in diameter, but the length of the barrel of the gun was 38 times times its bore, or 190 inches in length. That’s almost 16 feet!
The fleet ship I served in was armed with a 5″/54 caliber main armament cannon in the bow and two twin mount 3″/50 anti-aircraft guns in the waist.