Yeah, I've always had to mele after the Shotty to kill supers with less DR.

If I remember: Bubble and Well die to shotguns pretty much with no extra issue, Storm Edge and Behemoth are resistant and basically need every pellet to land but will die in one hit, pretty much every roaming super except I believe Arc Staff, Burning maul, and Dawnblade die to 1 shotgun + one melee as long as enough pellets hit and you're in range. Also depends on the shotgun type, overshield versus no overshield, and other dr sources but at a base you're right.