Following extensive trials, the German Rheinmetall 120 mm smoothbore gun (as installed in the German Leopard 2 MBT) was selected, with modification, for the General Dynamics Land Systems M1A1 and M1A2 MBTs. More recently the 120 mm M256 has been fitted into the two prototypes of the South Korean K1A1 MBT.

Production of the 120 mm M256 is undertaken at the US government owned Watervliet Arsenal. These are delivered to General Dynamics Land Systems for installation in the M1A1/M1A2 MBT turrets. Production of brand new M1A1/M1A2 MBTs has been completed but a programme to upgrade M1 MBTs with a new turret to bring them up to M1A2 standard has been underway for several years.

As a private venture, General Dynamics Land Systems is marketing the M60-2000. This is an upgraded M60 chassis fitted with the complete turret of the M1A1 armed with the 120 mm M256 smoothbore tank gun.

Description

The 120 mm M256 smoothbore tank gun consists of the gun tube, evacuator chamber group, breech operating group, breech ring group and breech block group.

The actual gun tube is a one-piece steel forging, cold worked with a smoothbore for the 120 mm ammunition.

The gun tube is attached to the breech ring by interrupted buttress threads and locked with a tapered stud so providing a quick change feature which permits removal and installation of the gun tube through the gun shield of the tank. This feature permits the combination gun mount, breech operating ring, breech ring group and breech block group to remain as a unit.

A smooth surface immediately forward of the interrupted buttress threads plus a front bearing adaptor provides the bearing surface for the gun tube in the recoil mechanism of the combination gun mount.

The evacuator group aids in removing spent propellant gases from the tube preventing the escape of these gases into the crew compartment. The annular chamfers at the front and rear accommodate O-rings to eliminate leakage of the spent gases. The breech operating group actuates the breech block to open and close the breech and move the extractors to eject the spent cartridge cases.

The breech ring group consists of the breech ring, closing mechanism bracket, extractor pivots and gun tube index stud. On the right side and at the rear of the breech ring a counterbored hole contains the firing pin plunger, firing conductor and sleeve assembly.

The firing plunger is energised by the firing contact assembly mounted on the recoil mechanism when a firing trigger is depressed. The electrical energy is then conducted through the firing conductor to the breech block firing contact to the firing pin assembly.

The breech block group is of the dropback, sliding wedge design that slides in grooved bearing surfaces in the breech ring assembly to open or close the breech.

The breech block group consists of the firing pin assembly with related parts and breech block. Machined cam grooves (extractor paths), located on the forward portion of both sides of the breech block actuate the trunnions of the extractors mounted on pivot pins projecting inward from the left and right side of the breech ring.

Flat surfaces (trunnion seats) are provided at the end of each patch for locking the breech block in the open position. When the trunnions of the extractors are positioned on the trunnion seats of the breech block, the breech block cannot.be closed until the extractors are tripped.

The electric firing pin assembly with firing pin spring is secured by a retainer assembly at the centre of the breech block rear. The contact, that conducts electrical energy to the firing pin, is housed in a hole, perpendicular to the firing pin, in the right side of the breech block.

The retractor driver, which is actuated by the breech block operating crank as the breech opens, forces the retractor guide and retractor rearward, withdrawing the firing pin assembly into the breech block.

The breech block has a concave surface at the top for ease of loading a round. A tapped hole in the centre of the concave surface is used for inserting an eyebolt to facilitate breech block removal. The upper portion of the front face is bevelled to provide the wedging action needed to properly seat the cartridges in the breech ring body as the breech block is being closed.

Production. In service with Egypt (M1A1), Kuwait (M1A2), Saudi Arabia (M1A2) and the US (M1A1/ M1A2). The 120 mm M256 has also been installed in prototypes of the K1A1 MBT developed for the South Korean Army.