WordNet
riot gun
n. a firearm designed to disperse rioters rather than to inflict serious injury or death
Wikipedia
Riot gun
In current usage a riot gun or less-lethal launcher is a type of firearm that is used to fire " "non-lethal" or "less-lethal" ammunition for the purpose of suppressing riots. Less-lethal launchers may be special purpose firearms designed for riot control use, or standard firearms, usually shotguns and grenade launchers, adapted to riot control use with appropriate ammunition. The ammunition is most commonly found in 12 gauge (.729 inches) shotguns and 37mm and 40 mm (1.46 and 1.57 inches) grenade launchers.
In the United States, the term riot gun more commonly refers to a riot shotgun.