The 140 mm smoothbore Advanced Tank Cannon (ATAC) System consists of the XM291 gun, the XM91 autoloading system and a family of 140 mm ammunition. The ATAC has an been installed in the Component Advanced Technology Test-Bed (CATT-B). This is based on an M1A1 Abrams chassis and, in addition to the ATAC System, has an Advanced Integrated Propulsion System (AIPS), hydropneumatic suspension, Standard Army Vetronics Architecture, Rockwell Multisensor Target Acquisition Sensor, CDC ADA-based fire-control system, lightweight track, modular armour, advanced chemical agent alarm, NBC collective protection system and a Vehicle Integrated Defense System with countermeasures, threat sensors and smoke.

Due to a shortage of funding, the CATT-B was never completed as a fully integrated system. Prior to being installed in the CATT-B, the ATAC System has been installed in a modified M1 turret on a standard M1 chassis. This also features the Benet Laboratory designed automatic loader, modified M1A1 fire-control electronics, Rockwell Multisensor Target Acquisition Sensor, standard M1 gunner's primary sight and a new fire-control computer. In this vehicle the gunner is on the right, as in the M1, while the tank commander is on the left in the space normally occupied by the loader.

The 140 mm XM291 gun is a solid propellant tank gun with an integral mount and recoil mechanism that fires two-piece (projectile and charge) ammunition with twice the muzzle energy of the standard 120 mm M256 gun installed in the current M1A1 and M1A2 Abrams MBTs. The ordnance is fitted with a thermal sleeve, fume extractor and is 91 kg lighter than the standard 120 mm M256 ordnance.

The system can also fire 120 mm one-piece conventional and advanced ammunition by means of a simple tube change, which takes one hour to accomplish.

During operation of the XM91 autoloading system, developed by the US Army's Benet Laboratory, the selected ammunition type is identified, the telescoped cell containing the cartridge is moved to the loading port and the loading door is opened. The rammer mechanism then moves forward and grasps the rim of the stub base, the round gripper mechanism which holds the cartridge in place is released and the inner cell moves forward.

Docking of the inner cell with the breech occurs on full extension and the round is seated in the gun. The ram head and the inner cell then retract to their original position and the loading door is closed. The downloading sequence is similar and an ammunition re-arm port is provided at the rear of the autoloader for that procedure.

The ammunition transfer mechanism has been developed by Lockheed Martin with the ready use bustle-mounted magazine holding 17 120 mm or 140 mm rounds, with the'hull magazine holding 22 140 mm two-piece rounds as components or 33 single-piece 120 mm rounds.

Automatic loading takes place at the rate of 8 to 12 rds/min, using a ribbon rammer.

The AT AC System family of ammunition consists of three cartridges: the XM964 kinetic energy (KE) cartridge; the XM965 chemical energy (CE) cartridge; and the XM966 training cartridge for both rounds.

Each cartridge consists of a forward and rear component. The rear component, which is identical to all three cartridges, consists of a stub base and primer assembly (similar to that of a standard 120 mm cartridge), a combustible side wall, an ignition system and propellant.

The forward component houses the appropriate projectile, partially telescoped within the primary propelling charge, and propellant in a combustible cartridge case which also contains a relay charge at its base for the transfer of ignition from the rear component. A snap joint joins the two components allowing cartridge removal from the breech

Prototypes undergoing US Army trials. At the present time there is no production application for the 140 mm Advanced Tank Cannon System.