my main backstop is at 300 yards out... it's a big box 30 ft wide, X 6 ft thick, X 10ft high... the boxes surface is 2 layers thick of treated 2 X 6's staggered so the seams of the under layer are covered by the middle of the top layer... then I put steel 55 gallon drums in front for target backers... I still was damaging the wood more than I liked in heavily shot areas... so I added 4' X 4' X 1/4" AR 200 plates screwed across the bottom of the backstop... & those get pretty cratered up by 30 caliber & larger rifle bullets
my pistol backstop is off the side of the rifle range & consists of steel 55 gallon drums bolted together 2 high & 3 wide, 2 deep on the bottom layer, & single deep on the top...it only took 2 years of heavy use, before the barrels got so shot out the dirt ran out, & I started shooting through them with my 45 Colt CAS loads
I'm wondering if old tires stacked & filled with gravel or dirt would have too much bounce back for CAS loads ???