Shotguns
Shotguns are classed according to gauge, a related expression. The gauge of a shotgun refers to how many lead spheres the diameter of the bore would equal a pound. In the case of a 12-gauge shotgun, it would take twelve spheres the size of the shotgun's bore to equal a pound. A numerically larger gauge indicates a smaller barrel: a 20-gauge shotgun requires more spheres to equal a pound; therefore, its barrel is smaller than the 12 gauge. This metric is used in Russia as "caliber number": e.g., "shotgun of the twelve caliber." The sixteenth caliber is known as "lordly" (Russian: барский). While shotgun bores can be expressed in calibers (the .410 bore shotgun is in fact a caliber measure of .41 caliber ), the nature of shotshells is such that the barrel diameter often varies significantly down the length of the shotgun barrel, with various levels of choke and backboring.
Read more about this topic: Caliber