riot gun
A style of shotgun characterized by it's light weight, short barrel (18.5 - 20"), open-choke, pump-action feed/eject mechanism and is almost always 12-gauge in caliber. Such a gun is typically relatively cheap because there are a billion import brands that use this design and anyone who can scrape up a couple hundred bucks can acquire one. Due to it's design, a riot gun is easy to maneuver, maintain and operate. Loaded with buckshot, a riot gun (or any shotgun) will "remove flesh and bone" at close range. Very powerful. For increased range and/or penetration of thicker/tougher targets, a riot gun will fire any manner of slug no problem with impressive results.

The drawbacks to a riot gun include heavy recoil (due to it's heavy payload and light weight), limited ammo capacity, short range and relatively weak armor penetration (with buckshot). Overall with proper ammo, a riot gun is excellent choice for close-range combat and even if you don't kill your target with the first shot, you'll probably slow/stun/knock them down enough to get the fuck out of there or hit them again.
The Winchester 1897 in riot/trench style could be considered the first riot gun. With a 20-inch barrel, this gun killed the fuck out of whoever tried to enter an American battlefield trench in World War 1. The gun was so effective that Germany tried to have it banned from warfare but was ultimately unsuccessful.