Let me repeat my earlier suggestion: Use sand for backstops! One feet of sand will stop any handheld firearm (except, maybe an RPG)
Bales and stacked wood may be alright in a pinch, but they will quickly develop a "tunnel" if you keep firing at the same spot - not so with sand, it will just keep swallowing those bullets.
This guy did some actual testing:
https://www.theboxotruth.com/the-box...sands-o-truth/