I put in a new range last year at my place, and my solution to the backstop problem was to build 10" deep 42"wide by 48" tall sand boxes , hang them on 6X6 posts with railroad ties stacked 16' wide and 10' high behind the posts, with sand filled semi truck tires behind the railroad ties.
The boxes allow me to reclaim the lead for recasting and I only have to replace 16" square chunks of plywood in the center when it gets completely shot out.I am sure that somewhere down the road I will have to replace the rest of the ply, but not for years yet.I put truck mudflaps behind the plywood as a gasket to keep the sand in, and they have worked great so far.I replace the plywood centers when 8" or so are completely missing, and have done this 3 times on the middle box and the original mudflap is still doing it's job.I have yet to see anything penetrate to the back of the boxes, everything stops or disintegrates in the first 6" or so including tungsten tipped 5.56,.243 win, .300 win mag and 7.62.The 9MM and the .40SW with the exception of the rifling almost look good enough to reload, the rifle bullets are in small fragments.
Here are some pics to give you the idea, though a few of these are reposts from another thread.
I had to put up a protective panel to keep the bullet splatter from chewing up the barn siding when shooting the clanger plates.
Nothing will get through this.( I still had 2 more tiers of tires to go at this point)
A new patch and the removed piece.
From 75 yds and the following is from 250 yds.