The 120 mm Armoured Mortar System (AMS) comprises a 120 mm smoothbore breech loading, recoiling mortar integrated with a full solution fire-control system and mounted in a lightweight steel turret that can be fitted to a wide variety of light armoured vehicle chassis.

The then BAE Systems, RO Defence first developed the 120 mm breech-loading mortar as a private venture in 1985. The first prototype of the mortar, complete with its elevating mass, was completed and test fired in mid-1986. During 1987, the turret system was integrated onto a now BAE Systems Ground Systems (previously United Defense) M113A2 chassis and test fired.

In the autumn of 1991, the now General Dynamics Land Systems - Canada Light Armoured Vehicle (LAV) (8 × 8) was fitted with the system and successfully completed initial trials at a UK weapons range. During these trials around 150 rounds of 120 mm mortar bomb were fired at high and low angles in the indirect fire mode as well as in the direct fire mode. For the trials, the turret was also fitted with elevation and traverse drives and a mock-up of a fire-control system.

BAE Systems, RO Defence was responsible for the turret and 120 mm ordnance, General Dynamics Land Systems - California Technical Center of the US was responsible for the electronics and sighting system and General Dynamics Land Systems - Canada was responsible for the LAV (8 × 8) chassis and turret integration.

During 1992, General Dynamics Land Systems - Canada/BAE Systems, RO Defence undertook the design and construction of a complete turret system with integrated fire control that they then demonstrated. The system comprised a new all-welded steel turret, the 120 mm ordnance and a complete fire-control/sighting system integrated on an 8 × 8 LAV chassis.

In late 1995, another 120 mm AMS turret was produced and then fitted to the private venture the now BAE Systems Ground Systems Mobile Tactical Vehicle Light chassis, a further development of the widely deployed M113 series. This system was then subjected to extensive trials and a customer demonstration in a country in the Middle East.

Early in 1996, the then General Dynamics Land Systems - California Technical Center awarded the then BAE Systems, RO Defence a contract worth GBP37 million for the supply of 73 120 mm AMS and associated ammunition.

Under the terms of the contract, BAE Systems, RO Defence supplied the turret, including the 120 mm mortar, General Dynamics Land Systems - California Technical Center supplied the computerised fire-control system and sights.

The complete turret was then fitted to the General Dynamics Land Systems - Canada Light Armoured Vehicle (LAV) (8×8) chassis.

The BAE Systems, RO Defence order included a substantial quantity of 120 mm ammunition that was developed by MECAR of Belgium and includes high-explosive (Composition B), illuminating and smoke (white phosphorus) natures.

The AMS has also successfully carried out a series of firing trials at a UK range with the RUAG Land Systems 120 mm mortar cargo round which is designated the 120 mm Ka G 98 in Swiss Army service.

The Swiss 120 mm mortar cargo bomb is already in quantity production for the Swiss Army and contains 32 grenades, each of which has a High Explosive Anti-Tank (HEAT) warhead that will penetrate 70 mm of steel armour. It also has a good fragmentation effect. A key feature of this grenade is that it has a mechanical impact fuze with a self-destruct mode and self-neutralisation.

Under a Foreign Military Sales (FMS) agreement, General Dynamics Land Systems - Canada has supplied a total of 1,117 LAVs in 10 versions to the Saudi Arabian National Guard (SANG). The 73 120 mm AMS variants are within this figure of 1,117 vehicles.

General Dynamics Land Systems - California Technical Center closed down in late 2004. RO Defence is now BAE Systems Land Systems. The 120 mm AMS is no longer marketed.

The 120 mm mortar has a semi-automatic conical screw, swinging breech mechanism with obturation being achieved with a Crossley pad. Firing is percussion mechanical activated by a solenoid. The recoil mechanism consists of two buffers and a pneumatic recuperator.

Sustained rate of fire is 4 rds/min, rapid rate is 8 rds/min (sustained for 3 minutes maximum). The ability to fire in direct mode at targets in excess of 1,000 m provides the ground force commander with enhanced operational capabilities while also improving the speed of response to conventional indirect fire roles.

The mortar can fire all standard 120 mm conventional smoothbore ammunition including the new generation smart bombs currently under development. With the 120 mm high-performance mortar ammunition, ranges in excess of 9,000 m were achieved. Also, as the weapon can be fired in the indirect mode of fire at much lower angles of elevation than a conventional mortar, it is much more difficult to detect with mortar locating radar.

The turret is fabricated from armour steel and has stations for two crew. The commander is to the right and the loader to the left. Each crewman has a hatch for access and egress, and an array of vision periscopes. Mounted either side of the turret is a bank of four electrically operated smoke grenade launchers which fire over the frontal arc and mounted on the turret roof is a 7.62 mm machine gun for the commander.

The complete system is all-weather capable and can revert to manual back-ups should a power failure occur. The turret traverse and elevation drives are hydraulically operated.

The full solution fire-control system employs a differential Global Positioning System (GPS) aided Turret Attitude Sensor System (TASS). The latter continuously and automatically updates location, turret bearing and tilt and cant parameters to the fire-control system. This allows the AMS quickly to engage the enemy with into action times of under a minute being possible. With the information and target location data stored in the computer, re-engagement of any target from any location within range limitations requires only the selection of that target from the computer menu. A number of targets can be entered into the system to allow rapid switching between them.

For the direct fire role a thermal sight assembly with integral laser range-finder is fitted.

Further development by the now BAE Systems Land Systems has resulted in the 120 mm Turreted Mortar System (TMS) covered in a separate entry.