Jim, that has to be the best (most practical) idea for a bullet trap that I have seen. With the growing difficulty of getting wheel weights..for free.. I think that there is one of your designed traps in my future. I will have to be careful not to let one of the guys I shoot with use the trap though...he has shot the legs of my target stand 3 of 5 times I have let him shoot at it, and it's 16'' wide.
Thanks for sharing the results of your experiment.