A bullet trap,which is a fantastic piece of kit BTW.....can be as simple or "engineered" as desired.
First up;inside or out?
Next;portability?
Then budget.Personally,I'd price out factory units as a data point.We'll assume there aren't a plethora of "used" units laying around to be picked up for beer money but hey,who knows?
We still have a "baby" snail that aside from a few bumps N bruises still works as well 30+ years later.Short story time.....a cpl years ago,I conspired with #3 son.Took the BS (baby snail) up to the fourth floor here at the estate.#3 has his cell on a decibel ap,sitting next to woman of my dreams,on the couch(2nd floor) watching some dumb .... on the tube.We didn't tell her,but at such N such time,I sent a cyl full of 38 sp into the BS.
His db meter didn't pick it up,and swmbo never broke conversation.Not recommended,just sayin....a little testing goes a long way.
Indoor use is more about vent system ($$$) than anything else in my pea brain.Chop a whole in a basement foundation and use the exposed dirt as a berm.Again,not recommending.But with primo vent system,that's pretty simple.
We're using 2' diameter Oak stumps outside,layed so we're shooting at the end.Ain't perfect but,dang cheap and will stop a 338.