Shotgun shell: A compact cartridge, usually made out of plastic, designed to be discharged in a shotgun. Shells come in many different sizes, or gauges. The smaller a gauge number/size, the larger the shell.

Shell casings are different colors, depending on their gauge. The most common gauge shells are 12 and 20-gauge shells, colored red and yellow, respectively. The largest gauge currently in existence is the 4-gauge. The shell is approximately 1 inch in diameter.

Composition of a shell:
At the base, the primer is seated and fixed in the brass. Then, the powder is loaded. Next, a wad is inserted (usually made of plastic). The purpose of the wad is to hold and cushion the shot as it is expelled from the barrel. The shot is then placed in the shell, and the top of the shell is then crimped, (much like the rolls of dimes and quarters you get from the bank). This design makes it easy to reload shotgun shells.