Thanks for the suggestion, I love the idea of making this mod compatible with other things, as it's really easy to do, the hardest part is the graphics.
The 75mm shell is the base-game cannon shell, so I am making that remain the same. Then I'm roughly planning to make them follow the progression of power:
Auto-cannon
75mm
88mm
128mm
Atomic bomb (the rocket basically)
So the difference between each one is the same. From a very brief play test, this seems to work fairly well, although the fire-rate of the super-heavy tank is awful, and the rocket-tank is stunningly devastating, until it inevitably kills itself.
I am also making a super tiny nuke for the auto-cannon, which works fairly well. Only the thermobarics now need doing, then I'll do another update.
