The M203 is an American-made single-shot underbarrel grande launcher designed to attach to a rifle, chambered for 40×46mm grande rounds. It uses the same rounds as the older stand-alone M79 break-action grenade launcher, issued to American forces during the Vietnam War. Though versatile, and compatible with many contemporary rifle models, the M203 was originally designed for the US standard-issue M16 and its variants. Stand-alone variants of the M203 also exist.
The M203 was designed to replace the M79 grenade launcher; the M79 was a stand-alone device and therefore usually the primary weapon of troops who carried it, and because the size and weight of 40mm ammunition limits the quantities that can be carried on patrol (and because a grenade is often not an appropriate weapon for a given engagement, such as when the target is at close range or near friendly troops), an under-barrel system was conceived as a more versatile replacement. Most obviously, an underbarrel launcher has the advantage of allowing its user to also carry a rifle, and to easily and quickly switch between the two.
The M203 has been in service since 1969 and was introduced to US military forces during the early 1970s. It fires the same rounds as the M79 that it replaced. When affixed to a full-size M16, the weapon requires a distinctive ventilated handguard to be attached to the weapon, replace the rifle's standard cylindrical handguard; attaching the launcher to smaller varaitns such as the Colt Commando or M4 Carbine simply requires the removal of the bottom half of the rifle's standard handguard. Mounting systems for a wide array of other rifles have since been developed.
M433 High-Explosive Dual Purpose (HEDP): The HEDP round has an olive drab aluminum skirt with a steel cup attached, white markings, and a gold ogive (head of the round). It penetrates at least 5 cm (2 inches) when fired straight at steel armor at 150 meters or less, or, at a point target, it arms between 14 and 27 meters, causes casualties within a 130-meter radius, and has a kill radius of 5 meters.
M406 High-Explosive: The HE round has an olive drab aluminum skirt with a steel projectile attached, gold markings, and a yellow ogive. It arms between 14 and 27 meters, produces a ground burst that causes casualties within a 130-meter radius, and has a kill radius of 5 meters.
M583A1 Star Parachute: This round is white impact or bar alloy aluminum, with black markings. It is used for illumination and signals and is lighter and more accurate than comparable handheld signal rounds. The parachute attached to the round deploys upon ejection to lower the candle at 7 feet per second. The candle burns for about 40 seconds. A raised letter on the top of the round denotes the color of the parachute.
M585 White Star Cluster: This round is white impact or bar aluminum alloy, with black markings. The attached plastic ogive has five raised dots for night identification. The round is used for illumination or signals. It is lighter and more accurate than comparable handheld signal rounds. The individual stars burn for about 7 seconds during free fall.
M713 Ground Marker: This round is light green impact aluminum with black markings. It is used for aerial identification and for marking the location of soldiers on the ground. It arms between 15 and 45 meters. If a fuse fails to function on impact, the output mixture provided in the front end of the delay casing backs up the impact feature. The color of the ogive indicates the color of the smoke.
M781 Practice: Used for practice, this round is blue zinc or aluminum, with white markings. It produces a yellow or orange signature on impact, arms between 14 and 27 meters, and has a danger radius of 20 meters.
M651 CS: This round is gray aluminum with a green casing and black markings. Though it is a multipurpose round, it is most effective for riot control and in Urban Operations. It arms between 10 and 30 meters and produces a white cloud of CS gas on impact.
M576 Buckshot: This round is olive drab with black markings. Though it is a multipurpose round, it is most effective in thick vegetated areas or for room clearing. Inside, it has 20 metal pellets, each weighing 24 grains, with a muzzle velocity of 269 meters per second. The round has no mechanical-type fuse.
In Predator, Dutch has an M203 grenade launcher affixed to his M16. After losing most of his equipment fleeing from the Jungle Hunter, he creates makeshift explosive spears and bombs from two remaining rounds for his M203.
- In reality, most of the M203 grenade launchers seen in the films of the Alien, Predator and Alien vs. Predator franchises have not actually been real. In Predator, the M203 used by Dutch is in fact a fake mock-up with a 39mm bore diameter. This particular model of fake grenade launcher was popular in action movies of the 1980s. The M203s seen in Aliens vs. Predator: Requiem, meanwhile, are in fact Cobray 37mm flare launchers, another common stand-in for the M203 that is frequently seen in movies. In both cases, alternative weapons were likely used simply because of the difficulty and expense in obtaining a real military-issue grenade launcher. Only Predator 2 features a genuine M203.