I was at a shooting range once where the backstop was conveyor belting. This 1/2" carcass coal mine rubber belting, 48" or 60" width. This placed in front of an earthen berm. I noticed that 9mm rounds would not penetrate it, lodging in the rubber. And it took quite awhile for it to be shot up as it was sort of "self healing."
Elvis Pressley used two layers of railroad cross ties, one layer run horizontally, the other vertically. Worked for him.