OS OK, it does not surprise me to see you have a very good reply to this question. I see quality in what you do and how you do it. I'll bet you were a darn fine electrician.
A very much younger me did a lot of bullet catching trying to learn about cast hunting bullets. ...Jacketed too, as a bases of comparison. My primary bullet catch setup was 4 one gallon water jugs lined up in front of a cardboard box of old clothing (rags). The setup would catch everything I shot at it including 375 H&H. The water is a much harder medium than game flesh like elk, and the rags act like a catchers mitt to put and end to the greatly reduced velocity. Old blue jeans really work well. Hint - if anyone tries this, put the box of rags in a large trash bag to keep the box from getting wet and soft -falling apart.
The rubber mulch idea is a good one!
OS OK - A side note: I was a professional firefighter but moonlighted as an electrician. Shortcuts and poor work I find in the houses I have owned and friends houses make me grumpy and shake my head. I like quality work.