The XM174 was an experimental 40mm automatic grenade launcher born from the fighting during the Vietnam War. The design emerged in 1968 from the Aerojet Ordnance Manufacturing Company and attempted to mate the form and function of the Browning M1919A4 air-cooled .30 caliber machine gun with the firepower of the 40mm grenade. The repeat-fire function would allow gunners to suppress enemy forces or dislodge dug-in foes through direct or indirect fire. The weapon was tested exclusively during the Vietnam conflict but was never formally adopted by US armed forces.
The XM174 retained much of the appearance of the original M1919A4 machine gun including its rectangular receiver, obvious charging handle and rear-set grip handle. The principle visual change was integration of the large barrel assembly intended to project the 40x46mm low-velocity grenade. This naturally forced a reworking of the internals of the M1919 system and ammunition was fed from a 12-round detachable drum magazine affixed to the left side of the receiver. Spent shell casings were ejected through a port along the right side. Sighting was through an iron flip-up aperture ahead of the receiver. The XM174 recorded a rate-of-fire of 350 rounds-per-minute firing from a basic recoil-induced action. Overall weight of the system was 7.25 kilograms.
The XM174 was trialled through several methods - as a portable crew-served implement and as a vehicle-mounted support weapon. For the former, the weapon was tied to a heavy-duty tripod assembly which supported traversal and elevation. The pointed feet allowed the weapon to be firmly dug into the soil, two legs reaching rearwards and a single leg at front. This then required the crew to sit behind the weapon in a conventional fashion or fire from a semi-prone position if under fire themselves. For the latter method, the weapon system was fitted to a pintle mount to allow for trainable fire. This meant that the XM174 could be installed across both land vehicles and aircraft such as helicopter gunships. In either case, the system provided a hefty amount of ranged firepower though the ammunition supply was rather limited. The XM174 was such a weapon that it could be used in an offensive or defensive manner as well, allowing for greater tactical flexibility.
The US Army, USMC and USAF Security Police elements all evaluated the type in service. Throughout the war, the 40mm M79 single-shot, break-action grenade launcher proved the norm for US forces. other developmental alternatives were issued though none formally accepted in number. The M79 eventually graduated to become the underslung 40mm M203 of today and the automatic grenade projecting system of choice went on to become the belt-fed Saco Defense Mk 19 first used in 1968.