Isn't the arming core where the plutonium "hammer" lies?
Or have nukes changed?
What I mean is this...
Little Boy was basically an encased gun, with a chamber with uranium at the end, and to set it off in mid air (it exploded some 1,440 feet up to create a "crush" effect on the buildings below), the plutonium fell from an upper chamber into the other material, thus setting off the explosion.