It is widely confused as a licensed production of the Rheinmetall L44 tank gun, however it was developed by IMI from 1983 to 1988, to meet the requirements of the Israel Defense Forces' Merkava Mark III main battle tank, i.e. a narrower mount like in the M60, both tanks designed around a 105 mm gun.

It was first revealed in 1989 when it was shown to be the main armament of the Merkava Mark III.

This tank gun is very similar to the Rheinmetall L44 versions installed on the German Leopard 2, American M1 Abrams, South Korean K1A1, and Japanese Type 90 main battle tanks.

However, it has a different recoil system, consisting of an optimized concentric retarder and pneumatic recuperator, and more compact overall dimensions, not exceeding those of the existing 105 mm M68 rifled gun of the Merkava Marks I, II, and the M60 tank.

There are three variants of the IMI 120 mm gun:[3] the MG251, fitted with a thermal sleeve developed by Vishay Intertechnology and provided with a fume extractor which can be removed for maintenance without disturbing the actual sleeve, its improved version MG251-LR[3] and the MG253, with a thermal sleeve developed by VIDCO Industries and a new compressed gas recoil system.