It had to be able to fire all 40 mm low-velocity grenades but be loaded from the breech to accept future longer projectiles.

[7] After the U.S. Army at Picatinny Arsenal conducted a competitive bidding process for a new 40 mm grenade launching system, Heckler & Koch was awarded a contract to provide the XM320 beginning in 2006.

[3] The weapon was officially fielded in July 2009 at Fort Bragg by the 1st Brigade Combat Team, 82nd Airborne Division.

[9] A grenadier carrying an M320 with an M4 and three dozen 40 mm grenades will have a total weapon load of 38 lb (17 kg).

[14] The M320 could also use the AN/PSQ-18A, which is an enhanced aiming device designed to enable rapid and precise fire in daylight, low light, and night conditions.

To replace the AN/PSQ-18A, the U.S. Marine Corps and U.S. Army separately adopted their own advanced sighting systems tailored for their needs.

The Marine Corps fields the SU-277/PSQ Grenade Launcher Sight (GLS), which features a holographic red dot for improved aiming, integrated ballistic data for automatic adjustment based on ammunition, infrared aiming laser, and a compact, lightweight design.

Selected in 2019 after years of development and testing, the GSS incorporates a reflex optical sight with integral backup iron sights, a near-infrared (NIR) aiming laser for night engagements, and an NIR illuminator for increased target recognition and identification.

Advanced sensors for temperature and pressure enable precise ballistic compensation, while an indicator for left or right tilt, also known as cant, further improves accuracy during firing.