A shotgun shell, shotshell, or shell is a type of rimmed, cylindrical (straight-walled) cartridges used specifically in shotguns, and is typically loaded with numerous small, pellet-like spherical sub-projectiles called shot, fired through a smoothbore barrel with a tapered constriction at the muzzle to regulate the extent of scattering. A shell can sometimes also contain only a single large solid projectile known as a slug. The hull usually consists of a paper or plastic tube often covered at the base by a metallic head cover which retains a primer, and the shot charge is typically contained by a wadding/sabot inside the case. The caliber of the shotshell is known as its gauge.
A 12-gauge shotgun shell in a transparent plastic hull, allowing the contents to be seen. From left to right: brass, propellant, over-powder wad, shot wad, #8 birdshot, over-shot wad, and crimp
Shotgun shell comparison (left to right): 12-gauge, 20-gauge, 16-gauge, 28-gauge, and .410 bore
M35 .410 shotgun shells for M-6 survival gun w/.22 long rifle for comparison
CCI .22LR snake shot loaded with #12 shot
A rim is an external flange that is machined, cast, molded, stamped, or pressed around the bottom of a firearms cartridge. Thus, rimmed cartridges are sometimes called "flanged" cartridges. Almost all cartridges feature an extractor or headspacing rim, in spite of the fact that some cartridges are known as "rimless cartridges". The rim may serve a number of purposes, including providing a lip for the extractor to engage, and sometimes serving to headspace the cartridge.
Rimmed .357 Magnum revolver ammunition
Rimmed vs Rimless cartridges
Rimless 9mm Parabellum pistol cartridges