The M203 grenade launcher can be fitted to a variety of non-M16/M4 rifles, increasing its global use.

The M203 40mm grenade launcher is a single-shot system that was designed to be attached under the barrel of an assault rifle or carbine. The addition of the M203 to any rifle system expands the capability of the standard infantryman to an extent. The M203 was debuted in 1969 and entered service shortly thereafter. Before then, the US Army relied on the stand-alone M79 single-shot grenade launcher - especially in the Vietnam War - but limitations for the operator were soon apparent (the operator had nothing more than his pistol as a secondary weapon). Likewise, the experimental XM148 single-shot underslung grenade launcher (fitted to an M16 rifle) was another viable option at the time, with this system being field tested with mixed results in the Vietnam Conflict in an effort to supply the operator with a better secondary weapon other than a sidearm. The M203 inevitably went on to fill this void and replace both grenade launching systems in use and under consideration.

The M203 was initially developed for use with the M16 series of rifles and the M4 series of carbines. The system was attached to the forward part of the gun, just under the barrel. The trigger was part of the launcher though no pistol grip was fitted. Instead, the magazine of the weapon doubled as the grip. Sights were provided by the manufacturer and were operated separately from the standard rifle sights.

The M203 is a multi-faceted weapon as showcased by its many operators across the globe. The weapon can fire eight distinct 40mm grenade rounds including the M406 HE (High-Explosive) round, the M433 HE DP (High-Explosive, Dual-Purpose) armor penetrator round, the M585 White Star Cluster illuminator round, the M651 Tactical CS crowd control CS gas round , the M781 Practice round for training, the Star Parachute illuminator/signaler in white, green and red colors, Ground Marker in red, green and yellow for marking targets or positions and the Buckshot round for close-quarters room clearing.

The M203 weighs in at just 3 pounds and raises the overall weight of a standard M16A2 series rifle (8.79lbs) to an acceptable operating weight of just under 12lbs. An effective precision range of 492 feet is reported while an area out to 1,148 can be targeted. The maximum range of the M203 is listed at 1,312 feet. The M203 also sports a minimum effective range of just over 100 feet. M203 units are shipped with the leaf sight, quadrant range sight and a hand guard.

Variants of the M203 include the base M203 for use with the M16A1, M16A3 and M16A3 series of rifles as well as the M4 and M4A1 series of carbine systems. This model features a 12-inch barrel. The M203A1 is offered for use on the M4 and M4A1 and features a shorter 9-inch barrel length. The M203A2 was developed for use with the M16A4 MWS (Modular Weapon System) and sports a 12-inch length. The M203 PI was developed to fit the M203 grenade launcher to other non-M16/M4 systems. The M203 had proven so popular for its time that foreign weapon companies were forced to design indigenous M203 attachment options for specific rifles and submachine guns.

Though the Mk 19 automatic grenade launcher also fires the 40mm round, it is of a slightly different caliber (40 x 53mm) and fires at a higher velocity than the 40mm (40 x 46mm) round used in the M203.

The new M320 single-shot, double-action, side-loading grenade launcher is set to replace the M203 in service with US military forces.