ATRs weren't as a general rule very effective. This represents the machineguns or small arms available to the crew. Of course if you have HE capability, why fool around with the little stuff. Again, the rules lack clarity on the subject. I don't have my cards handy.
Some guns have greater explosive capability than others of the same caliber.
But please give me specifics and I'll try to address them. I'm really glad you're enjoying TC, and am truly sorry that you play has been impeded by these nagging rules questions.