“Would .40 (.400”) ball stop any better than 9mm (.355”) ball? I don’t think it would make that much of a difference, and with 9mm you get more rounds in the same length magazine.”
It a HUGE difference because diameter isn’t the only measure of effectiveness.
I know the kenetic energy difference between .40 and 9mm, but with ball ammunition you’re most likely going to get a passthrough and a majority of that energy is not delivered into the target.