The problem I'm visualizing is, "What do you do if you miss the trap?"
For a testing range, the test barrel/receiver is probably bolted down and there just isn't any chance of the shot doing anything
but entering the bullet trap.
But if you're shooting from a bench, there is the chance of missing the bullet trap. So you need a backstop rated for pistol/rifle rounds, including height and width to allow for "stray shots".
This is where it starts to get expensive.
I've seen some pretty good ranges with oversized steel plates, typically set at a 45° angle, and a sand bed beneath. And, for pistol work at closer ranges, there was a "lip" to protect the shooter from the sand that kicks up. But they still had a substantial earth berm behind the angled plate.
I've also seen indoor ranges with a similar setup, plus a steel barrier closer to the firing line to prevent shots from "going over" the bullet trap. (The steel barrier was probably a hazard to other shooters, but it eliminated downrange hazard, and it was close enough that you were afraid of hitting it.
)
If the steel is the correct alloy (usually armor steel), it won't crater and create a hazard by having an impact surface that isn't at a 45° angle.
I believe setups like this can handle pistol at close range, and rifle at 100yds minimum.
