Nitpick: "caliber" has several different meanings, all of which (confusingly) relate to a gun's barrel dimensions. The one you're using is a measure of internal barrel diameter, essentially a shorthand for inches (i.e. .22 caliber); the decimal point often gets dropped in that context, though. It's equally correct to speak of caliber in terms of some other unit, like millimeters. When you're talking about large weapons, though, the word means the length of the weapon's barrel as a multiple of its internal diameter; a tank gun might be 120 mm 55 caliber, making it 6.6 meters long.

The American customary system doesn't as far as I know have a general-use length unit in the millimeter range. There are a couple of typographical units defined in terms of the customary system, but they haven't really escaped into the wild.