Perhaps you could have something like the main gun on the AMX 13, which is a hugely popular and sucessful post-war French light tank with an autoloader. It comes in 75mm and 90mm variants and is very light, all while packing a six shot revolving autoloader for the 90mm version and twelve shots for the 75 mil. It fires very fast and from what I gathered from research, was very reliable and accurate. Maybe you could do something like that instead?