|Guns and Ammo|
Since the advent of the cartridge ammunition its measurement and description of size of the ammunition has been caliber (calibre), sometimes confusing to the new shooter.
The caliber is the internal diameter of the barrel as well as the diameter of the projectile, called the bullet. Calibers of firearms are also referred to in millimeters, originating in Europe – now used by US military to keep standard ammunition amongst the NATO armed forces.
Shotgun shells are not measured in caliber, but in gauge. The gauge of a shotgun refers to how many lead spheres, each with the diameter of the bore, amount to one pound in weight. So a 12-gauge shotgun would take 12 spheres the size of a shotgun's bore to equal one pound. The larger the gauge the smaller the barrel, so a 20-gauge takes 20 spheres to make a pound. Shotgun bores can be measured as calibers, for example, the .410 bore shotgun is .41 caliber (11 mm). Barrel diameter will vary down the length of a shotgun barrel with various types of choke and backboring.
Artillery barrels are described in multiples of the bore diameter, for example, a 4-inch gun of 50 calibers would have a barrel 50 x 4 that equals to 200 inches long. A 50-caliber 16-inch naval gun (16-inch shell) has a barrel length from the muzzle to the breach of 66 ft, 8 in.
The following video, produced by the National Shooting Sports Foundation – NSSF – explains handgun calibers and the importance of shooting the right ammunition meant for your firearm.