Yes there is but you don't see it! It's based on caliber,I would think with your extensive knowledge you'd have known that.
No, because its just anything above 40mm doesn't count as gun, according to the arena flags. But neither do they count as ordnance. For reasons that are apparently arbitrary, HTC has just decided that the BK 5 and 75mm on the B-25H are in some other category of their own, and I would assume use hard coded ammunition limits.
Thats not consistent with the rest of the system, it just means that those are the only two things that are out of order.
