if you are concerned about splatter, then use a 60 degree angle. Also, use 3/8 or 1/2 inch plate, but don't screw the plate down. Let the plate lift up or " bounce " when hit by a big slug, and you will do much less damage to the plate. I recommend 1/2 " plate for those big slugs, and balls. Any thing less, and you eventually get bowling, where the plate gets a big dent from repeated blows. It takes a lot of muscle and sweat to pound those bowls out of the plating with a sledge hammer, and it gets harder and harder to find volunteers to do that kind of work.
Make a steel frame to hold the back plate, but let the plate lift. If you can find rubber conveyor belt material to use a baffles, to slow the balls and bullets, by all means use them. They will be much easier to replace than the large back plate. Put sand at the bottom of the " V ", to catch the balls, and go ahead and use 1/4 " steel for the side walls to contain splatter in the " box ". Some folks use a half sheet of plate( 4' x 4' and angle it from the bottom of the box up to a point where it meets the backplate The idea is that balls that are deflected downward will hit this tillted base, and bounce around forward and land in the sand for easy retrieval. The angle is usually somethingin the range of 10-15 degrees sloped upward, rather than the steeper 60 degree angle of the back plate. You can also leave a gap between the back edges of the two plates, and then weld a Steel pipe cut open to accept the lead spatters, and bullets to the two plates. That makes a true bullet trap, Like the old Detroit Bullet Traps, only one for rifles.
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