Shotguns can be acquired in several ways. They may be sold in weapon shops, and also have a low chance of being found inside a crate.
All Shopkeepers carry shotguns to enforce order in their stores, but you will need to fight them in order to take it and will be branded a wanted criminal by doing so.
A guaranteed shotgun can be found in a Restless Dead level, buried under a unique grave with 'ASH' inscribed upon it - a reference to the protagonist of the Evil Dead movies.
The shotgun fires buckshot, discharging 6 pellets at once that each deal 4 points of damage. If you or an enemy are damaged by a pellet, there is brief delay before additional pellets will do damage. The pellets are fired on slightly divergent trajectories and spread slightly as they travel, but this doesn't really affect long-range accuracy. There is a brief 'reloading' time after firing that restricts its rate of fire, but the weapon has unlimited ammunition.
Most enemies will be killed with a single shot, but large, tough monsters can withstand several hits.
Some enemies are immune to gunfire, including the Ghost, Olmec, Croc Men and Bacteria. The Black Knight's Shield will reflect your bullets back at you, but he will be vulnerable if you can make him drop it by stomping on him.
Remember that the recoil from firing a shotgun will throw you back by one tile's width. Always make sure that this won't cause you to fall off ledges or push you into a trap.
It is possible to jump one tile higher with the shotgun's recoil by firing and jumping simultaneously. This can be combined with the spring shoes to effectively double your natural jumping height.