An automatic shotgun is an automatic firearm that fires shotgun shells and uses some of the energy of each shot to automatically cycle the action and load a new round. It will fire repeatedly full auto until the trigger is released or ammunition runs out. Automatic shotguns have a very limited range, but provide tremendous firepower at close range.
Automatic shotguns generally employ mechanisms very similar to other kinds of automatic weapons. There are several methods of operation, with the most common being gas, recoil, and blowback operated: Gas operation uses the pressure of the gas (created by the burning propellant) behind the projectile to unlock the bolt assembly and then move it rearward. Recoil operation uses the backward force applied by the projectile (due to Newton's Third Law of Motion) to retract the bolt assembly. Blowback operation uses the backward force to retract the entire barrel and bolt assembly, which unlock at the rear of the barrel's path. Each of these methods use springs to return the retracted parts to their forward positions and restart the cycle. Many automatic shotguns are capable of selective fire, meaning they can fire in multiple modes (semi-automatic, three-round burst, and sometimes fully automatic).
They generally store ammunition in detachable box or drum magazines in order to decrease reloading time, whereas most pump-action and semi-automatic shotguns use under-barrel tubular magazines. Shotguns are able to handle a wide variety of ammunition, however automatics are slightly limited because the shot must provide sufficient force to reliably cycle the action. This means that they are not compatible with low powered rounds, like most less-than-lethal ammunition. The most common ammunition used in combat shotguns is 00 buckshot, which fires about 9 metal balls and is very effective against unarmored targets.
A standard shotgun shot fires multiple small projectiles at once, increasing the chances of hitting the target. Shotguns have a short effective range of about 50–70 metres (160–230 ft), but provide a lot of firepower at close range. Automatic fire enhances these effects, due to the increase in the rate of fire. Automatics typically have much shorter barrels than pump-action shotguns (especially hunting shotguns). Barrel length contributes to tighter shot spread, so automatics have relatively wide spread. Short-barreled automatic shotguns have a very high chance of hitting close range targets, and can even hit multiple targets in one area, which is ideal for combat situations. Long-barreled pump action shotguns are more accurate and have increased range, which is ideal for hunting and sporting purposes. Automatic shotguns are generally viewed as less reliable than manual operation shotguns, because there are more moving parts and increased chances of error. If any one piece fails, it will most likely halt the operation and cause damage to the weapon and/or user. Automatic weapons are also more susceptible to jamming and negative effects from dirtiness.
Automatic shotguns are intended for use as military combat shotguns. They typically have a high rate of fire and relatively low recoil, making them ideal for engaging multiple targets in a fast paced combat situation. They are able to fulfill several different combat roles, due to the wide variety of shotgun ammunition available. A single gun can be used to breach a door, clear a room, provide support with special grenade rounds, or even suppress a riot with some less-lethal rounds. Automatic shotguns have not seen much use in the United States, but have been slightly more popular in some other countries.