@RamscoopRaider, good point. Perhaps we can go a different route with the tanks and have one that is like a more conventional tank but with a bigger engine. So a medium battle tank that is faster and a bit larger as a result. Got a decent punch but the extra speed allows the tanks to group together in larger groups much faster for a defeat in detail or allows each tank to more effectively intercept an enemy attack. The extra speed would also allow formations to choose their battles, such as retreating when they are losing and having their enemy not be able to catch up. The speed means that enemies will find it hard to retreat. Same conventional medium tank gun and armour, extra cost and bigger profile (perhaps looking like a heavy tank due to the large engine size) but very fast.
About the 4mm bullet, thanks for that.
On another note when you look at a bullet the gunpowder granules seem to have a bit of air space between them. Would it be worth it to pour some gunpowder like substance into the cartridge so it perfectly fills the cartridge with no air spaces? The point is to potentially increase the power of a piece of ammunition without changing the dimensions at all by removing all the air spaces. You could also try having ammunition with the same power but is now able to be smaller due it having no airspaces inside the cartridge. You could use the stuff they use on caseless ammunition which seems to be able to be moulded into shapes.