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.
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.
AC4 tank