Let's re-evaluate. My first experiment was with about 13-14" of moderately packed, standard rubber mulch as found at Walmart. It stopped 6 full 158gr JHP loads and did not penetrate the 13-14". My plan is to use another minimum of 4" packed more firmly, and back it with a 3/16" AR400 steel plate. I doubt any bullet that manages to penetrate the 18" of rubber would even dent the plate, but would then be flattened, or redirected into more rubber, or both. I have not recovered bullets yet, as I didn't particularly have a reason, but now I will, just to see if they mushroomed. I realize hardcast WOULD penetrate further, but that is not my desire (to find things that WILL defeat my bullet trap). This is a starting point, and it looks as though an effective trap can be built easily from materials costing about $250-$500, depending on if you want a 2'x4' trap, or a 4'x4' trap. Add another $200-$300 for an effective ventilation system and ductwork, and it is starting to look very good.