What I really wanted to do was weld a 3 inch dia. 10 inch long steel pipe that I would've cut a slice in length wise to those steel plates to make something like this but my crude plywood box works lol
Edit: You don;t really need the plate on top or at the bottom since your bullet travels in a straight line even after hitting one of the side plates, it just follows the plate to the opening. with the pipe even if you were to shoot in the slit you would hit the curved inside of the plate and the slit is made on the side (see pic above) so your bullet just follows the circle till it has no more speed