The one I used to shoot at had a heavy guage steel backdrop. Don't know how thick it was just because I never cared to ask. What I do know is that it has to be angled (top towards the shooter) so that the bullet fragments go down into a trap. The trap is usually 3 feet or so high and the top is angled toward the backdrop. Make sense? If you're planning on building one, make sure to do your homework. I'd hate to think what could happen if you did it wrong. The Discovery Channel did a show on this about a year ago. Search their website, they sell the videos. As for the forum, this probably would have done better in the gun forum.