The M203 was the only part of the army's Special Purpose Individual Weapon (SPIW) project to go into production. The M203 has been in service since 1969 and was introduced to U.S. military forces during the early 1970s, replacing the older M79 grenade launcher and the conceptually similar Colt XM148 design. However, while the M79 was a stand-alone weapon (and usually the primary weapon of troops who carried it), the M203 was designed as an under-barrel device attached to an existing rifle. Because the size and weight of 40 mm ammunition limits the quantities that can be carried on patrol, and because a grenade is often not an appropriate weapon for a given engagement (i.e. when the target is at close range or near friendly troops), an under-barrel system has the advantage of allowing its user to also carry a rifle, and to easily switch between the two.
A new grenade launcher, the M320, will eventually replace the M203 in the United States Army. The United States Marine Corps, Air Force, Coast Guard, and Navy will continue to use the older M203. The M320 features an advanced day/night sight, a double-action firing mechanism (as opposed to the M203's single-action) as well as other benefits, such as an unobstructed side-loading breech.
There are numerous variants of the M203 manufactured in the U.S., and throughout the world, for various applications. These vary chiefly in the length of the barrel, attachment type, and quick detach (QD) capability.
The standard M203 is intended for permanent attachment to the M16A1, M16A2 and M16A3 rifles, and utilizes a 12-inch rifled barrel. The M203 unmodified to an A1/A2 series will not fit on the M4 Carbine series.
The U.S. M203A1 has a barrel of 12 inches, while the SOPMOD M203A1 has a 9-inch barrel. The M203A1 is intended for use with the M4 and M4A1 Carbine, and uses a special bracket mount consisting of three screws and lacing wire. Only M203A2's consist of a Quick Release Bracket.
The Canadian M203A1 by Diemaco (now Colt Canada) is a similar design with a different mounting system that does not require mounting points of the same profile as the M16A1 rifle's. The weapon's 9-inch barrel slides further forward than the standard American models, which allows longer rounds to be loaded. This model is identifiable by the increased distance between the grenade launcher's barrel axis and the rifle's. This weapon may no longer be in production, but is still in use.
The M203A2 is intended for use with the M4 Series/M16A4 and now also authorized on the M16A2 Rifle as the MWS (Modular Weapon System). Using standard 12-inch barrels, the grenade launcher is intended for use in concert with the Knight's Armament Company M5 RAS. The M5 MWS Rail System became authorized in December 2008 for the M16A2 Rifle. An advantage of this system is the use of range-finding optics to make precise targeting easier.
The M203PI system is used for attachment of the M203 to other rifles, including, but not limited to, the Steyr AUG, Heckler & Koch G3 and other rifles, and even the MP5 sub-machine gun. Most of these other companies have since devised 40mm grenade launchers custom integrated with the weapon.
The M203 DAX has a double-action trigger and longer breech opening to accommodate less-lethal rounds.
The M203 and M203A1 are currently manufactured by AIRTRONIC USA, Inc. of Elk Grove Village, Illinois for the U.S. Department of Defense under contract numbers W52H09-06-D-0200 and W52H09-06-D-0225. Each contract is for up to 12,000 units. Each unit is shipped with hand guard, leaf sight and quadrant range sight. The contracts unit prices vary from $840 to $1,050 each. The production rate is 1,500 units per month. The M203PI is manufactured for both the U.S. Department of Defense and for commercial sales to law enforcement agencies both in the United States and abroad, and for foreign military sales by RM-Equipment Inc. of Miami, Florida.
The Turkish MKEK made T-40 grenade launcher is based on the M203.