For many years the US Army deployed two self-propelled mortar (SPM) based on a modified BAE Systems, Ground Systems Division M113 series full tracked armoured personnel carrier (APC).

These were the 4.2 inch (107 mm) M106/M106A1/M106A2 and the 81 mm M125/M125A1/M125A2. These have now been replaced in US Army service by the BAE Systems, Ground Systems M1064A3 120 mm self-propelled mortar system.

The chassis used for the M1064A3 120 mm self-propelled mortar is essentially the late production M113A3 APC modified for its specific role.

The hull is of all-welded aluminium armour, which provides the occupants with protection from small arms fire and shell splinters with the highest level of protection over the frontal arc.

The driver is seated at the front of the vehicle on the left side with the diesel power pack to the right. The driver is provided with a single-piece hatch cover that opens to the rear, and to his front and side are four M17 day periscopes with an AN/VVS-2 image intensification or AN/VAS-5 thermal viewer provided in the roof.

The diesel power pack compartment is to the right of the driver's position and is fitted with a fire-extinguishing system that can be operated by the driver or from outside the vehicle.

The air inlet and outlet louvres and the exhaust pipe outlet are in the roof and there is an engine access door in the front of the hull that hinges forwards.

The power train consists of the power pack (engine, transfer gear case and transmission) steering control differential, pivot steer, final drive and associated drive shafts and universal joints. The diesel fuel tanks are mounted externally either side of the hull at the rear and are armour protected.

In the middle of the roof, to the rear of the driver's position, is the commander's cupola, which is provided with vision blocks and a single-piece hatch cover that opens to the rear. On this is mounted a .50 (12.7 mm) M2 HB machine gun that is used for local or air defence purposes. Mounted either side at the front of the hull is a bank of electrically-operated smoke grenade launchers that cover the frontal arc.

The crew enter the vehicle through the large power-operated ramp at the rear, which is also provided with a door for emergency use.

The torsion bar suspension either side consists of five dual rubber-tyred road wheels with the drive sprocket at the front and the idler at the rear. The first and last road wheel stations are provided with a hydraulic shock absorber. There are no track return rollers.

A rubber track shroud on either side of the hull controls the flow of water over the tracks when the vehicle is afloat. The M1063A3 is fully amphibious and is propelled in the water by its tracks. Steering when afloat is the same as on land.

Before entering the water the two bilge pumps are switched on and the trim vane, which folds back onto the glacis plate when not in use, is extended at the front of the hull.

For its specialised role the floor features a welded-in crossbeam and additional floor-support structure to withstand mortar reaction forces.

Mounted in the rear is the 120 mm muzzle-loaded mortar, which fires through a three-part roof hatch. The mortar is normally fired to the rear through a 90° arc.

A baseplate is carried on the left side of the hull, which allows the mortar to be dismounted for ground use if required by the tactical situation.

Types of 120 mm ammunition fired by the M1064A3 self-propelled mortar include the following:

• 120 mm high-explosive M933

• 120 mm high-explosive M934 (as M933 but with different fuze)

• 120 mm smoke WP M929

• 120 mm M930 illuminating

• 120 mm M983 infra-red illuminating

• 120 mm practice M931

The M120 mm M933 has a maximum range of 7,240 m and a minimum range of 300 m. Maximum rate of fire is 15 rounds per minute and sustained rate of fire is 4 rounds per minute.

BAE Systems, Ground Systems is offering a wide range of optional equipment for this vehicle including a more powerful Detroit Diesel engine, more powerful generator, automatic fire detection and suppression system in the engine compartment, enhanced protection against direct-fire weapons and mines, air conditioning system, various types of land navigation system and mortar fire-control systems.

BAE Systems, Ground Systems can supply kits to convert M113, M106 and M125 vehicles to the latest M1064A3 configuration.