We used to shoot all sorts of x mils into a pile of logs with dirt covering ....What you do need to be careful of is "tunneling".....shift your target around ,and check the backstop isnt getting a big hole almost thru.......I would think a 45deg angled plate of a 360 or plus grade steel 1/2" thick would stop even steel core milsurps at 100yds.....However ,in this age,be careful of splattered/vaporized lead causing you EPA type pain in the future.................more recently ,I have been using discarded carpet as a backstop.12 ft of carpet edgewise will stop any bullet,even a 50 ,I would think......Unfortunately ,the stuff will rot if its not plastic carpet,and get smelly.....But the stuff is free.,and delivered free ,too.