Gauge and Gun Size
The gauge number is a fraction of one pound in weight. A ball the same width as a 12-gauge barrel would weigh one-twelfth of a pound and a corresponding ball for a 20-gauge would be one-twentieth of a pound. The actual diameter of a 12-gauge barrel is 0.729 of an inch and a 20-gauge is 0.615 of an inch. Therefore, the 12 is a wider gun, and it takes a broader cartridge.
A 12-gauge has a larger cartridge; therefore, it can deliver a heavier payload than a 20-gauge. Shotgun cartridges contain either "Shot" (small lead balls) or "Slug" (single lump of lead). Slugs are rarely used and are illegal in many countries, such as in the UK. There are many different types of shot size depending on the game. Bird shot is small (about 0.15 inch), and a 12-gauge cartridge may hold up to 340 lead balls in one bird shot cartridge, allowing for a wide spread. Heavier shot -- such as buck shot -- is for larger game, such as wolves and small deer. There are only a few lead balls per cartridge of buck shot.
Weight and Kick
A 20-gauge is a smaller and lighter gun than a 12-gauge. When fired, it delivers a smaller kick (recoil) into the shoulder; thus, it is more suitable for a smaller sportsman or woman or a younger person.
The 12-gauge is one of the most popular sporting shotguns, because of its versatility and killing power at close range. Over half of shotguns and cartridges sold are 12-gauge. Although not as popular, the 20-gauge has the advantage of being lighter, and is more popular for upland shooting, on account if its lighter weight.