Development of the Rheinmetall 120 mm 44 calibre smoothbore gun began in 1964, although it was not until 1974 that the first hardware trials took place. Series production began in 1979. The gun barrel is a cold drawn tube, with the interior chrome-plated to increase the wear resistance and so lengthen the barrel life.

The barrel has a Glass-Reinforced Plastic (GRP) fume evacuator 2,500 mm behind the muzzle and the thermal sleeve is also made of GRP.

The breech is secured to the barrel by a threaded bayonet connection and employs a downward-opening semi-automatic sliding wedge block. The breech mechanism incorporates a hydraulically operated loading mechanism which positions the round on the loading chute. When the weapon recoils it is brought back into position by a recuperator. At the same time the breech block opens automatically. A semiautomatic loading system was developed but the only in-service installations to date (Leopard 2 and M1A1/ M1A2) use manual loading for the projectiles and the semi-combustible cases.

Firing is electrical. The recoil mechanism comprises two hydraulic retarders and a single hydropneumatic (hydraulic oil/nitrogen) assembly. Types of Rheinmetall-developed ammunition that can be fired are HEAT-MP-T, HEAT-MP-TP, APFSDS-T and APFSDS-T-TP, all of which have a partly combustible case with a metal base stub.

So far Rheinmetall has developed four generations of APFSDS-T ammunition for this weapon: the DM13, DM23, DM33 and DM43 which has been developed in association with Giat Industries of France and can also be fired by the Leclerc MBT. This has a penetrator with an increased length-to-diameter ratio for increased armour penetration characteristics.

The latest APFSDS-T round to be developed by Rheinmetall is the DM53.

Sweden is fielding an HE round rather than a HEAT-MP round; this is being developed by LIAB (a subsidiary of Bofors) and Israel Military Industries.

The 120 mm ammunition fired by this weapon can also be fired from the 120 mm smoothbore gun installed in the French Leclerc, Italian Ariete, US M1A1/M1A2, Japanese Type 90, and Israeli Merkava Mk 3 MBTs.

In production. Installed in the Leopard 2 MBT; M1A1 MBT (first production vehicles delivered 1985) and the M1A2; the US version is designated M256. Rheinmetall is now offering the 120 mm smoothbore gun for retrofitting to other MBTs such as the US M60 series and the Leopard 1. First trials of a Leopard M1A1 fitted with a 120 mm Rheinmetall smoothbore gun were carried out in late 1983 and early in 1985 a Leopard 1 with this gun was successfully demonstrated to current Leopard 1 users; this vehicle also had the EMES-18 fire-control system. The Leopard 1 fitted with the 120 mm smoothbore gun has a total of 42 rounds of 120 mm ammunition: 29 in the hull and 13 in the turret. The Rheinmetall 120 mm smoothbore gun is also manufactured under licence in Japan for the Mitsubishi Type 90 MBT. Rheinmetall is also developing a 120 mm low recoil concept similar to that of the 105 mm 105 SLR system. This will be suitable for installation on tracked and wheeled vehicles weighing only 18 tonnes.