The M203 grenade launcher is a lightweight, single-shot, breech-loaded, pump action, shoulder-fired weapon that is attached to the AR-15 series rifles and carbines. It fires a 40mm (40x46mm) grenade round.
The standard M203 utilizes a 12" rifled barrel is intended for permanent (armorer) level attachment to the M16A1, M16A2, and M16A3 rifles, but can also be fitted to the M4 and M4A1 carbines using a different front attachment point forward of the front sight block.
The American M203A1 is intended for use with the M4 and M4A1 Carbine. The barrel is shortened to 9", and principally the M203A1 QD is able to quickly detach from the rifle, and be replaced by a Knight's Armament Company M4 RAS lower handguard.
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" barrel slides forward further than the standard American models to allow longer rounds to be loaded. This model is identifiable by the increased distance between the grenade launcher's barrel axis and the rifle's.
The M203A2 is intended for use with the M16A4 MWS (Modular weapon system). Using standard 12" barrels, the grenade launcher is intended for use in concert with the Knight's Armament Company M5 RAS.
The M203PI system is used for attachment of the M203 to other firearms such as the Steyr AUG, H&K G3, and even the MP5 sub-machine gun. Most of these other companies have since devised 40mm grenade launchers custom integrated with the weapon.
The RAMBO (Rapid Additively Manufactured Ballistics Ordnance) is a 3D printed variant of the M203.