This is my air rifle backstop. The steel is 3/16" and 19" x 18" and on about a 45 degree angle.
There is a 2" angle iron base that the bottom of the plate sits on. This not only protects the wood,
but collects the pellets in huge chunks of welded together projectiles.
So far, the largest thing I've shot at it are .45 cal bullets from my big bore air rifle and highest speed
are pellets running just over 1,100fps.
The design is what I am showing and a thicker plate could be used.
I like the 2 designs above. I used a wooden one at Lester Hawkes "diggins" back in the 70's. He just burned the wood
and the lead would melt out and puddle on the concrete floor of his shop. It was easily retrieved in the morning before the
stove was fired up again.