I used setup of 50 litre can filled with 25kg of rubber mulch. 5mm steel plate behind can for safety. All stuff in 60x60x100 box filled with rockwool, at least 15-20 cm each side. Were quite.
You can make a cheap bullet trap with a Home Depot 5 gallon bucket and rubber mulch. It will stop small calibers and give you something safe to shoot at. As far as reducing noise I’ve just used an FTN and subsonic ammo. Not perfect but it helps. Let me know if you come up with anything interesting.
In addition to this, hanging some old blankets surrounding your test area may be of marginal help, the thicker the better.
Build a box out of plywood, insulate around it with rockwool, and build another box on the outside of that, also insulated. Should be a good start at least. Don't forget the backstop!
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