Originally Posted by ArmchairGeneral

I'd bet money it's a mortar round. Too small to be an air dropped bomb, even a hand dropped WWI, in my opinion. I would be surprised if they made bombs 2 inches wide and 4 inches long. And the shape of the fins definitely suggests it was tube launched. My first though was the German 5 cm mortar of WW2, but the round isn't shaped right, and it's too short. Most WW2 rounds seem to be tear drop shaped, with a highly tapered rear. I'm looking up WWI mortars right now.