AC4 tank

The Australian Cruiser Tank Mk IV was a cruiser tank designed in Australia in World War II as the intended successor to the AC3 Thunderbolt. Like its predecessors the AC4 was to have a one piece cast hull and turret. The AC4's most important characteristic would be the use of a 17 pounder tank gun.

History
Reservations about the utility of the 25 pounder in the AC3, and the 25 pounder's limited ability to pierce armour led to experimentation with a 17 pounder mounted on an Australian cruiser.

A turret was built and mounted on one of the earlier development vehicles to assess the vehicle's ability to mount the foremost Allied anti-tank gun of the day – the British 17 pounder (76 mm, 3 in). This was achieved by mounting two 25 pounder gun-howitzers which when fired together would significantly exceed the recoil of a 17 pounder. In this configuration the tank was tested on 2 November 1942. It fitted with a 17 pounder and after successful gunnery trials on 17 November 1942 the 17 pounder was selected for the AC4 design. For the AC4 the 17 pounder was to be mounted in a new and larger turret, attached by a 70-inch (1778 mm) diameter turret ring, the space for which was accommodated by changes to the upper hull permitted by the compact nature of the "Perrier-Cadillac".

A design for the tank had been established, however it was subject to a redesign to alter the internal stowage, and include new features not previously considered such as removal of the turret basket, addition of a gyro-stabiliser, and swapping a hydraulic traverse for the electrical system, and torsion bar suspension for the volute spring used up until that point.

The programme was authorised to build a total of 510 AC4 tanks. The design was not yet finalised when the programme was terminated in July 1943.