It's a dud spigot morter round! *:

The tube in the rear slipped over a solid steel rod, with the propellent charge inside. Since you didn't have to worry about the bore of the tube, you could make the fins as large as you wanted. It's rather like a gun turned inside out, where the bullet stays put and the barrel is lauched. See a description of the British PIAT anti-tank laucher.
I suppose you could drop it from an aeroplane, but even the standard *60mm and 81mm rounds have fins. *Little bags of propellent are attached to the fins, called increments, and can removed to adjust range.