The M203 is a grenade launcher of US origin. It was designed in the late 1960's to replace the M79 in US service. The M203 is the first succesful under barrel grenade launcher and was designed to fit onto the M16A1. As a result the grenadier has a full size assault rifle as secondary weapon instead of relying on a pistol.

The M203 is a breech loaded grenade launcher that uses the same ammunition as the M79 it was intended to replace. In order to load the M203 the barrel is moved forward and the round is inserted. The weapon is cocked and ready to fire when the barrel slides back in position. The M203 mounts on the M16A1 just in fromt of the magazine. The magazine also acts as a sort of pistol grip as the single action trigger assembly is located behind the barrel. The launcher is attached to the rifle by means of a wider square forearm with various holes for cooling. The flip up mechanical sight is fitted on top of the forearm. Unlike the original M203 later models are quick detachable and will fit to more types of rifles.

The M203 is a single shot weapon that fires 40x46mm low velocity grenades. Besides the standard high explosive fragmentation rounds there is a wide selection of other rounds available. Due to the distance that the barrel slides forward upon reloading not all types of grenades can be loaded due to their length. An experienced operator can fire 5 to 7 rounds per minute. The effective range against point targets is 150 m. Area targets can be engaged out to 400 m.

The fist user of the M203 was the US military and it was used during the later stages of the Vietnam war and all subsequent conflicts. At first the M203 was only used by operators of the M16 series of assault rifles. Since the M203 became available in versions that will mount on other types of weapons its use expanded even further. The M203 is common in North and South America, Asia and Africa.