Realitycrash said:
Well you could technically say "assault gun", and that would kind of be right, since a gun is mainly meant for the purposes of "assault" (ie attacking). However the joke is playing on the fact that the more common and accepted term is "assault
rifle". Also, the container for the ammunition of an assault rifle (if you're to assume Erin was actually meaning "assault rifle") is typically referred to as a "magazine" or "mag". The term "clip" is typically reserved for pistols.
Shotguns fire ammunition stored in the shells that are loaded into the shotgun. At a super basic level, the shell is the casing holding the ammunition to be fired, the wad (separates the gun powder from the ammunition itself, and also helps the ammunition (particularly loose buckshot) stay coherent as it travels down the barrel), the gun powder that combusts and produces the energy to move the ammunition, and the primer (which contains a small amount of material that combusts when hit by the firing pin, thus beginning the combustion of the gunpowder proper).
Shotgun shells need not be limited to buckshot, you can put many different ammunition types in the shell. A popular ammo type for shotguns is a slug round, firing one single, aerodynamic, solid projectile. More exotic ammo includes stuff like "dragons breath", which basically fires fire.
Would like to get an enthusiasts take on it, since I'm really only a layman.