Des,
The sheet metal is 3/16 but the curved piece is 5/16 pipe and this is where the bullet hits the hardest as it moves around the curve and drops out of the bottom and into the box. I shoot 38 special loads into the trap when it is outdoors and doesn't seem to hurt a thing, but 22 shorts and 22 lr in the shop. I use a heavy mud flap hung in the throat of the trap to shoot through to keep any splatter from bouncing around. Please take all safety precautions should you decide to build one. Shooting firearms is risky, and I do not want anyone getting hurt. Thanks for the reply.