A typical 12 gauge shotgun slug is a blunt piece of metal that could be described as an 18 mm.In the slug are lots of tiny balls one of these balls are about 1/12-pound of lead. A shotgun is usually designed to be fired from the shoulder. The most common shotgun today is none other than the 12 gauge.

Now for the 20 gauge, this gun is a smooth bore shotgun that fires a shell that is smaller in caliber (.615 in.) than a 12 gauge shotgun (.729 in.)

A 20 gauge is more suitable for hunting certain types of game.