I built a steel trap out of a piece of 30" oil field pipe about 30" long. I cut it in half lengthwise then set it facing upward, like a water trough. I then welded a piece of 3/8 deck plate to one edge with it angled in at a 45 then put a lip on the edge to leave a "mouth" facing the shooter. I also closed in the ends of the pipe with some 1/4 plate and welded a piece of angle on the front edge of the opening to form an interior lip.
When a target is hung across the face, the bullet hits the deck plate and angles down, hitting the pipe and bounces around in sort of a half circle. Usually the lip catches everything but occasionally, there will be small holes in the targets from fragments. We have shot at this at 25 yds and nothing has come back at us. The bullets are pulverized and only lead dust and pieces of jacket remain. However, a buddy of mine shoot a 300 Win Mag and he has punched through the deck plate several times so I would use 1/2 or 3/4 if I built it again. I put pipe legs on it so the thing stands about bench rest height. It's quite heavy so we move it around the field with a tractor.
I now also have a permanent bullet dump made from railroad ties and dirt. We stacked the logs in a shallow V and dumped dirt in the opening. So far, this has stopped everything shot but of course, I have to dig the bullets out of the dirt. Good luck with whatever you do but make it plenty heavy if using big cartridges.
