A .22 caliber rifle fires a bullet with a 22/100 inch diameter. A 9 mm pistol fires a bullet with a nine-millimeter diameter.
Because the caliber is a machined value and very precise, specific ammunition is required for each gun.
It refers to the measure of the bore diameter of a shotgun. With the exception of the .410-bore, which is not a gauge at all (it’s actually a caliber) but often mistakenly called one, the gauge number is equal to the number of lead balls of that bore diameter that add up to weigh one pound. To get technical, a 12-gauge, the most common shotgun gauge today, is the diameter of a ball of lead weighing 1/12-pound of lead, while a 20-gauge is the diameter of a lead ball weighing 1/20-pound of lead.
From the smallest to the largest gauge, the list includes; 28-gauge, 20-gauge, 16-gauge, 12-gauge and 10 gauge. (Shotguns of long ago also included the mammoth 8-gauge and 4-gauge and the smaller 24-gauge and 32-gauge, but these are all collectors’ items now.) (source: National Shooting Sports Foundation)
Shotguns shoot, not surprisingly, shot. Patterns of pellets. But they can also fire the more lethal shotgun slug, which is a hunk of metal that is larger than most consumer rifles would fire. Generally, slugs are used to shoot big game including elk or bear.
The shotgun shell is the ammo on the right in the photo below.
Notice that some of the bullets have a flat head some are pointed. The difference is the intended purpose of the ammo. The flat round is called a “hollow point” and the bullet flattens out on impact causing more damage and stopping power than the pointed round that is meant to penetrate the target. The larger the casing (the brass part) the more velocity will be behind the projectile as it leaves the barrel.