We found a video of someone firing different rounds point-blank into a barrel filled with rubber mulch. This seems like the worst-case depth of mulch you’d need in a backstop for different bullet types (i.e. there would be less energy if you were firing from ten meters or a hundred meters, so there would be less penetration). I wanted to record the round types and approximate depth

22 long — 10″
9mm — 14-15″
40 cal — 18″
300AAC — 20″