I set 4 half telephone poles into post holes in a rectangle 5'x10', nailed treated 2"x6"s on the sides and back, and filled it with topsoil. The resulting 5' high, 5' wide, 7 1/2' deep (half way up the pile) backstop has been successfully catching bullets for me for about 20 years.
Every 3 or 4 years I add a front end loader of topsoil to replace what has splashed out.
