The M203 is a single-shot, reusable 40mm grenade launcher designed to extend the reach of the standard infantryman, allowing him to become a more balanced warfighter. The system is attached under the barrel of an assault rifle or carbine-type weapon and used without extensive modifications to the base weapon. The M203 debuted in 1969 and entered service shortly thereafter. Prior to the arrival of the M203, the US Army relied on the stand-alone M79 single-shot grenade launcher - particularly during the Vietnam War - but limitations for the operator were soon apparent: the operator had nothing more than a sidearm pistol as his secondary weapon. Likewise, the experimental Colt Firearms XM148 single-shot underslung grenade launcher (fitted to an M16 service rifle) became another viable option at the time and, although field-tested with mixed results during the conflict, the weapon failed to gain a larger foothold. This period of development ultimately led to AAI Corporation's M203 which subsequently replaced both the M79 and XM148 series.
The M203 was initially designed and developed specifically for use with the Colt M16 series of rifles and the related carbines of the time as these remained the standard issue firearms of the United States military. The grenade launching system was attached under the forward part of the gun, just under the barrel, and featured an integrated trigger unit. Since there was no pistol grip available, the operator would rely utilize the M16's magazine as the forward grip when actuating the M203. Iron sights for ranging were provided as standard and operated apart from the standard rifle sights.
As it saw more widespread use, the M203 proved itself a versatile weapon as showcased by its many operators across the globe. The weapon could fire eight distinct 40mm grenade projectiles 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. Each M203 weighed approximately 3 pounds and raised the overall weight of a standard M16A2 series rifle (already at 8.79lbs) to an acceptable operating weight of just under 12lbs. An effective precision range of 492 feet was reachable while an area out to 1,148 could be targeted depending on environmental conditions. The official maximum range of the M203 was 1,312 feet with a minimum effective range of just over 100 feet (the latter for safety reasons). M203 units were shipped from the manufacturer complete with leaf sights, quadrant range sight and the hand guard.
Variants of the M203 have included the original, 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 particular model featured a 12-inch barrel. The M203A1 was offered as a complement to the M4 and M4A1 and given a shorter 9-inch barrel length. The M203A2 was developed for use with the M16A4 MWS (Modular Weapon System) and fielded a 12-inch length. The M203 PI was developed to fit the M203 grenade launcher to 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, bringing an end to the M203's legacy in the US military inventory. Several other nations have also retired the type from service.
The M203 has been produced by AAI Corporation, Colt Defense, Diemaco, Airtronic USA, RM Equipment, US Ordnance, Knight's Armament Company and Lewis Machine & Tool Company. While AAI was the original producer of the weapon, they were not set up for large-scale manufacturing and thusly Colt was awarded the new production contract by the US DoD. AAI was responsible for only the first 10,000 units. Colt versions were faithful to the original AAI design.