The FN 303 is a semi-automatic less-lethal riot gun designed and manufactured by Fabrique Nationale de Herstal.

The FN 303 uses compressed air to fire projectiles from a 15-round drum magazine. It is designed to incapacitate the target through blunt trauma without causing critical injuries, and is most widely used for riot control and other kinds of combat where lethal weapons should be avoided.

The 303 can be fired from the shoulder using adjustable iron sights, or it can be mounted in an under-barrel configuration on most assault rifles when its stock assembly is removed (in this configuration, it is designated M303). It also comes with a top-mounted Picatinny rail that can fit most commercial weapon accessories, such as telescopic sights, laser sights and tactical lights. FN markets its own set of accessories for the launcher, including tactical vests, slings, carrying bags, and gas compressors. It is accurate at distances up to 35 metres (38 yd).

FN also produce a pistol variant, the FN 303-P, in the same caliber, using a seven-round magazine with a self-contained gas cartridge.

The FN 303 is based on a project by the Monterey Bay Corporation designated the XM303. The development team consisted of designers and researchers from two paintball related design and manufacturing firms, including Airgun Designs. The prototype was named the UBTPS, Under Barrel Tactical Paintball System.

The design could be attached to an M16 rifle and was conceived as a less-than-lethal weapon system coupled with a lethal weapon system, providing a wide range of response capabilities that were immediately available. A stand-alone version was also developed.

The UBTPS also featured a rotating barrel magazine, allowing for a wide range of different projectiles to be available and selectable without the need to change magazines.

Also developed was a bismuth-weighted projectile to provide the necessary mass, allowing the UTPBS to achieve the engagement range requirements.