Shotgun shell

A shotgun shell is the standard type of ammunition for a shotgun. It is fired after being loaded into the chamber or chambers of a shotgun. The shell is cylindrical in shape and is normally between and long. It holds a charge of gunpowder and either lead shot or a shotgun slug made to be fired from the shotgun. Shotgun shells are usually mostly plastic but have thin brass at the base, though they can be made of solid brass, paper, or other combinations of these materials.
A shotgun shell's size is usually measured in gauge (also known as bore). The most common size of shotgun shell is the 12-gauge. When measuring gauge, a smaller number means a larger shell: a 12-gauge shell is larger than a 20-gauge shell. Shell gauge is an old measurement. Except for the .410 bore or gauge, which is not a gauge at all but a caliber, 
the gauge of a shotgun was equal to the number of lead balls that added up to . So in a 12 gauge, 12 balls add up to about a pound of lead. So the diameter is equal to a ball weighing 1/12th of a pound of lead. A 20 gauge, another popular size today, has the diameter of a ball weighing 1/20th of a pound of lead.
Besides lead shot and slugs which are designed to be lethal, there are special less lethal types of shotgun shells which shoot things made to hurt but not kill people. Less lethal shells are most often used by police. These can be loaded with plastic or rubber shot, rubber slugs, beanbags, or salt.