I’m thinking a rounded/cylindrical tank might deform and spring back , the metal would be “self supporting” unlike a simple square steel plate would ... it could send the slug back with more force than you think ... time to experiment.
Been thinking about this one. At some point in its’ travel, the bullet’s velocity relative to the idiot was zero FPS. At the muzzle, .380 goes about 1000 fps, carrying 200 ft-pds of energy. ANY deformation of the slug, the tank, or movement by the tank is gonna eat some energy. If the bullet followed a radiused path around some part of the tank, there’s friction involved, but it wouldn’t have had to slow to zero fps in all vectors. Wonder what you’d get if you fired into a horseshoe shaped pipe.