The M203 grenade launcher entered service with the U.S. military in 1969 during Vietnam. It replaced the M79 "Blooper" stand-alone launcher, almost always being used as an under-barrel addition to an assault rifle.
Though it has served faithfully and effectively for over 40 years now and will continue to do so for years to come, the M203 is being phased out of Army service and is being replaced by the new M320 designed and built by Heckler Koch.
For now the Marines are sticking with the 203, though many top infantry advocates in the service want the Corps to replace its current ones with M320s.
The M320 won a competitive bidding process and entered production in 2008 with over 71,000 of the weapons planned for the U.S. Army. Soldiers of the 82nd Airborne Division were the first to field the weapon operationally.
While the M203 was capable of operating independently, in practice it is rarely used in standalone configuration. In fact, the old M79 resurfaced during Operation Iraqi Freedom as a superior option for grenade launcher duties without a rifle.
The break-action blooper (or 'thumper,' based on who you ask) was touted as a superior tool for the job when the whole rifle/launcher combo was too heavy or unwieldy, and standalone M203 units were not up to the task or simply unavailable.
The M79 has greater range and better accuracy than the M203. While it has performed admirably since Vietnam, no one has ever claimed that the M203 provided pinpoint accuracy.
The M79, introduced in 1961, is even older than the M203. Much more importantly, it's not capable of being used as an under-barrel launcher on an M4 or M16 rifle. While stand-alone launchers definitely have their place, the need for a grenadier who is also a rifleman is a crucial one in most cases. A new, better, under-barrel 40mm grenade launcher was needed.
The M320 filled that need. Using the same high-low propulsion system of the M79 and M203 to keep recoil low while firing a heavy 40mm projectile, the M320 has the same range as the M203 while increasing accuracy and coming with a number of improvements over the older model.
One of the most noticeable upgrades over the M203 is the M320's side-loading mechanism. The barrel swings out for loading, rather than the M203's forward-sliding pump-like barrel. This allows the use of additional, longer, ammunition, particularly non-lethal rounds. With the weapon's introduction, the Army is able to move forward with the development of new, high-tech rounds that wouldn't fit in the M203.
Another obvious feature of the M320 is the folding foregrip. The grip is intended primarily for use when the weapon is used separate from a rifle, but it can also serve as a forward vertical grip when mounted under a barrel. When not needed, the foregrip can be easily folded back and out of the way.
The sights of the M320 are certainly more advanced than those of the M203, and they benefit from being integral to the launcher itself, being mounted on the side of the unit. The M203's sights were attached separately and had to be re-zeroed every time.
The M320's leaf sight simply flips up when needed, and the integrated electronic sighting system allows users to dial in the range as determined by laser and tell if they're on target. This alone makes the M320 easier to field and more accurate in more conditions more of the time. While operating the M79 was an acquired ability and accuracy with the M203 was more art than skill, the M320's sight helps to make every operator a capable grenadier.
The M320 has a double-action trigger compared to the M203's single-action unit and has an ambidextrous safety. This allows the operator more control over his weapon, its firing, and better capability to handle a misfire or simple unloading.
Despite the M320's technical advantages over its predecessors, its introduction did not come without some hiccups. All new weapons systems suffer from some teething pains, particularly when introduced during a time of war, and the new grenade launcher was no exception.
While intended to be lighter than the M203, the M320 is actually slightly heavier, weighing in at 3.3 pounds compared to the M203's 3.0 pounds. While this difference is small, combat troops are already overloaded and every ounce counts.
While the new sight provides significant advantages over the M203's sight, some troops have complained that it's a little fragile for hard use in the combat zone. This may be due to the fact that the troops are used to not worrying about an M203 because there was so little to break.
Another complaint is that when used stand-alone with the stock assembly, the buttstock is a little short for many operators.
Finally, the single-point sling attachment of the stand-alone M320 meant that the weapon swung around and was often bouncing in the way, with troops calling for a holster of some sort to use while carrying the launcher unmounted. The Army responded by launching an M320GL Holster Soldier Enhancement Program.
The SEP was a "try-before-you-buy" program that used holsters from three different vendors and issued them to troops for testing and feedback. The holster solution will also address some of the concerns about the fragile sights, since the weapon won't be bouncing around or getting dragged on the ground when the operator hits the dirt.
A comment found online from a soldier claiming to carry an M320 in Afghanistan says that the launcher is a pain in the ass and swings everywhere, he "wouldn't trade it for anything else in a firefight." It's hard to come up with a better endorsement of the M320 than that.