Stand-alone variants of the M203 exist,<ref>http://www.knightarmco.com/m203_12ga.html</ref> as do versions designed specifically for many other rifles. The device attaches under the [[barrel (firearms)|barrel]], the launcher trigger being in the rear of the launcher, just forward of the rifle [[magazine (firearm)|magazine]]. The rifle magazine functions as a hand grip when firing the M203. A separate sighting system is added to rifles fitted with the M203, as the rifle's standard sights are not matched to the launcher. The version fitted to the Canadian C7 has a sight attached to the side of the launcher, either on the left or right depending on the user's needs.

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<ref name="hkusa01">http://web.archive.org/web/20070604003700/http://www.hk-usa.com/le_xm320_general.html</ref> and was introduced to [[U.S. Military|U.S. military]] forces during the early 1970s, replacing the older [[M79 grenade launcher]] and the conceptually similar [[XM148 grenade launcher|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&nbsp;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.