The M203 is a single shot 40 mm

40 mm grenade

The 40mm grenade is a military grenade caliber for grenade launchers in service with many armed forces. There are two main types in service: the 40×46mm, which is a low-velocity round used in hand-held grenade launchers; and the high-velocity 40×53mm, used in mounted and crew-served weapons. The...

 grenade launcher

Grenade launcher

A grenade launcher or grenade discharger is a weapon that launches a grenade with more accuracy, higher velocity, and to greater distances than a soldier could throw it by hand....

 designed to attach to a rifle. It uses the same rounds as the older M79

M79 grenade launcher

The M79 grenade launcher is a single-shot, shoulder-fired, break-action grenade launcher that fires a 40x46mm grenade which used what the US Army called the High-Low Propulsion System to keep recoil forces low, and first appeared during the Vietnam War...

 break-action grenade launcher, which utilize the High-Low Propulsion System

High-Low System

The High-Low system also referred to as the "High-Low Pressure system", the "High-Low Propulsion System", and the "High-Low projection system", is a design of cannon and antitank launcher using a smaller high-pressure chamber for storing the propellant...

 to keep recoil forces low. Though versatile, and compatible with many rifle models, the M203 was originally designed for the U.S. M16

M16 rifle

The M16 is the United States military designation for the AR-15 rifle adapted for both semi-automatic and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO...

 and its variant, the M4 Carbine

M4 carbine

The M4 carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16, all based on the original AR-15 designed by Eugene Stoner and made by ArmaLite. It is a shorter and lighter variant of the M16A2 assault rifle, with 80% parts commonality.It is a gas-operated,...

. The launcher can also be mounted onto a C7, a Canadian version of the M16 rifle which requires the bottom handguard on the rifle to be removed in order to mount the launcher.

Stand-alone variants of the M203 exist, as do versions designed specifically for many other rifles.