The indoor ranges use ground up tires like at the playground

I think one needs to consider what type of round is being fired and the distance.  My trap, currently at 50 ft, is based on 2x4 walls with chip board on both sides (think bullet catching out house) with road base fill in the walls.  Then I have two lawyers of angled steel plate (more resistant to puncturing) and the front is covered with heavy conveyor belting.  The lead ends up in the bottom of the house if it makes it through.   I normally fire just 22lr but the belting will stop 38 special rounds and the 45 ACP doesn't make even a dent on the steel.  The 22 rounds can just get stuck...not so good.  They sort of fuse together.   

I covered the house in some left over steel barn siding.  I need to paint a half moon on the front

I shot some low power 22 ammo and the bullets bounced back.  It may be overkill with the heavy belting. 

Dave

Double Dittos on the ground-up tire mulch.   This year I made one of a 55 gallon drum laid down for the back yard.   Half full of mulch, then a layer of heavy conveyor belting, then more mulch.  So far nothing has penetrated as far as the belting.  Face of the drum is Coroplast sheet.  Paper targets attach with push-pins.  This is set on a steep hill, so if I miss, the hill catches the bullet.  (I haven't missed it yet....   )

Bullets do not fragment.  Indeed some bullets could be reloaded and fired again.  

I bought seven bags of the mulch.  Only needed five.

Pieces about the size of the first joint of your thumb.  Plus or minus.  

BTW It's not really ground-up tires.  There's no bits of wire in it.