Further information in case anyone cares...
The 2lbr was considered obsolete with 6lbr on its way out. Aussie's knew about 17lbr but were not optimistic on getting any. The 25lbr was considered the next best choice.
The AC3 Thunderbolt would have had a 25lbr gun with a 18in longer barrel giving a L/34 barrel length instead of the standard L/28 barrel length. This would have bumped muzzle velocity to 2000fps (610m/s) with Stage 2 charge.
The AC3 25lbr Thunderbolt would have used a enlarged turret with extended turret bustle and turret ring diameter enlarged to 63 inches or 1600mm.
Engine was to be the three Cadillac setup with the 66th and later hulls using a Pratt and Whitney Wasp radial (renamed Scorpion for tank use) rated at 400hp. Instead Robert Perrier designed the Perrier Cadillac rated at 396hp which was three Cadillac V8's using a common crankcase arrangement similar to the Chrysler A57 multi bank engine. The Perrier Cadillac used less space, weighed less, and had much better low end power.
100 hulls had been built and production was underway when the program was terminated in July 1943. The Melbourne tank museum had a AC3 Thunderbolt assembled from the prototype and parts scrounged off target ranges. The Melbourne tank Museum closed in 2006 and the AC3's whereabouts are unknown.
Edited by Anlushac11, Mar 18 2017 - 19:33.