In various suspicious literature (think Friedrich Georg, Rob Arndt et al) the Barrel Bomb ("Fassbombe" in German) has been linked to every kind of Nazi secret last-days-of-the-war superweapon you care to name: nuclear, chemical, radiological, FAE, super-chemical, deep-freeze, etc..
To me, it always looked rather like a (air droppable ?) canister for supplies. And if you absolutely want it to be a weapon, my best bet would be an FAE or a firebomb (napalm or equivalent). This, I hasten to stress, is pure speculation on my part, however.
maybe it is a type of aerial mine? it would probably deploy a parachute of some kind out of the back?
i dont think it is a rato pod, as i doubt it would be mounted in the center of the arcraft like a bomb would. ???