Here's What You Need to Know: This grenade launcher is getting a bit old, but it's still a powerful weapon.

One of the most powerful weapons available to U.S. Army and Marine Corps infantry is a grenade launcher designed to fit underneath the M16 rifle.

The M203 grenade launcher gave soldiers unprecedented grenade range in a compact, durable package. The weapon, introduced in the waning days of the Vietnam War, is only now being replaced by a new, similar one.

In the late 1950s and early 1960s, the U.S. Army overhauled its infantry weapon arsenal. Passed over for updates as the service prepared for nuclear warfare, it gradually became exceedingly obvious that the M-1 Garand and other small arms left over from World War II were painfully obsolete, especially in light of Communist Bloc armaments such as the AK-47.

One of the new weapons designed to boost infantry firepower was the M79 grenade launcher. A single shot weapon that resembled an oversized break-open shotgun, the M79 could lob a 40-millimeter high explosive grenade up to 350 meters, or about where U.S. weapons such as the new M-14 started to lose effectiveness. The new M79 effectively gave platoons and squads their own handheld artillery, capable of lobbing explosive rounds far in excess of traditional hand-thrown grenade range.

The M79 proved a valuable weapon during the Vietnam War and validated the concept of the grenade launcher. Still, there were shortcomings. While a useful weapon, a squad or platoon issued such a weapon would lose the grenadier as a rifleman, and the grenadier usually only carried an M1911A1 handgun as a sidearm. The M79 traded aimed fire for area fire.

Why, asked the Department of Defense, couldn’t a single soldier have a weapon system capable of both aimed and area fire? The M79 had a short and stubby barrel, and perhaps there was some way to mount a grenade launcher under the new M16 rifle, specifically in front of the magazine well and parallel to the rifle barrel. A weapon that used existing M79 rounds and attached to the M16 would be ideal.

Several designs were offered up for the Pentagon’s under-barrel grenade launcher, including prototypes by AAI Corporation, Ford Motor Company, Colt, and the government’s Springfield Armory. Colt’s design, the CGL-4, advanced to become the XM148 grenade launcher. While the XM148 accepted M79 rounds it also suffered from numerous problems, including complexity, cracks in metal parts, sighting issues, and too much force being needed to cock the weapon. After some trials in the jungles of Vietnam, the service recommended withdrawing the XM148 and bringing back the M79.

The return of the M7 didn’t mean the service’s quest for an under-barrel grenade launcher was over, however. AAI Corporation, formerly Aircraft Armaments Incorporated, had developed a 40-millimeter pump-action grenade launcher not designed for the M16 but which could be adapted for the role. The weapon, known as the XM203, went up against larger, more complicated designs by the Aero-Jet Corporation, the Aeronutronics Division of Philco-Ford, and again, Colt. The XM203 handily won the competition based on “superior performance and lower cost” and was selected for adoption. The XM203 could also use M79 rounds, saving the military the hassle of developing and deploying new grenades.

The XM203—later M203—was a pump-action weapon that secured itself to a M16A1 rifle by way of a distinctive metal shroud that fit over the existing rifle handgun. The grenade launcher was loaded by pushing the barrel forward and inserting a 40 millimeter grenade into the breech. Pulling the handguard back cocked the weapon, and it was now ready to fire. The M203 had a separate trigger that launched the grenade, and a AAI-designed sight that worked with the M16 front sight post to aim the weapon. A short range weapon, it was often fired on a ballistic trajectory to hit targets at longer ranges. Six hundred XM203s were ordered for trial use in Vietnam.

The M203 passed trials and was accepted by the Armed Services in 1969. The M203 had the same range as the M79 and grenades had the same burst radius of approximately five yards. It weighed half as much as the M79 while still allowing a grenadier the use of a M16A1 rifle. A M203 grenadier could fire off his entire basic load of thirty-six rounds in less than three minutes, a devastating amount of explosive firepower for small infantry units.

A number of 40-millimeter grenades were developed for U.S. forces. The M381 and M406 anti-personnel grenades were filled with high explosive and featured serrations in the the body for fragmentation. The M381, with an arming range of 3 meters, was useful for urban combat while the M406 armed after 14 meters. The M433 High Explosive Dual Purpose round used a shaped charge warhead for use against vehicle armor but was also useful in the anti-personnel role. While the shaped charge warhead wasn’t powerful enough to penetrate tank armor it likely would have had utility against the armor of lighter vehicles such as the BTR-70 armored personnel carrier, BRDM-2 armored scout car and the flank armor of a BMP infantry fighting vehicle. The M651E1 tear gas grenade carried a payload of CS gas. Other grenades launched parachute flares, colored ground smoke, and white, green, and red flare clusters.

One of the most dangerous rounds for the M203 was the M576E1 Multiple Projectile. The M576E1 packed 20 rounds of .24 caliber buckshot. Developed for the M79, it effectively turned a grenade launcher into a shotgun with a deadly blast capable of shredding anything in its path. The M576E1 had a relatively short range but could shred anything within 50 yards downrange.

The M203 grenade launcher served the U.S. military for more than 30 years. The M203 took part in Vietnam, the invasion of Grenada in 1983, the UN peacekeeping operation in Lebanon in 1983, the invasion of Panama in 1989, operations in Bosnia, Somalia, and Haiti in the 1990s, and the 1991 Gulf War. After 9/11 iit went to war after in both Afghanistan and the invasion of Iraq. In addition to U.S. forces, it was generally adopted by any country that adopted the M16 series of weapons, including Australia, Canada, Greece, South Korea, South Vietnam, the Philippines, and (in limited numbers used by special forces) the United Kingdom.

In 2009, the Pentagon began the process of replacing the M203 grenade launcher with the M320 grenade launcher. The M320 features improved ergonomics, a lighter weight, safer operation, and can be used both as an under-barrel weapon or separate weapon system. U.S. ground forces are clearly committed to the grenade launcher concept.

While the M203 may soon leave service entirely it will be long remembered as the weapon that started it all.

Kyle Mizokami is a writer based in San Francisco who has appeared in The Diplomat, Foreign Policy, War is Boring and The Daily Beast. In 2009 he co founded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

This article first appeared several years ago.

Image: U.S. Marine Corps photo by Lance Cpl. Christopher J. Moore