12 gauge is the most common size of shotgun shell. Gauge is an very old method of measuring the caliber or interior diameter of a barrel. Essentially, it is defined as the number of lead balls, each of which will just fit inside the barrel, that are required to make up a pound. Consequently, the smaller the number of lead balls, the greater the diameter of the barrel. In this case, 12 lead balls, each about .73 inches (18.5mm) are needed to make a pound.