The #3 on 20 gauge ammo refers to the size of the shot (or projectiles) in the cartridge. Normally found as #3 steel shot, each projectile has a diameter of .14 inch and 154 pellets fit in a 10 ounce round. This shot size is typically used for large birds like geese, but it also bridges between birdshot and buckshot and, in some places, can be used in short-range deer hunting.