As a one time air gun competitor I have shot tens of thousands of pellets in my basement all at 10yds.
I shot into a steel box about 10"x10" backed with a clay like material called duct seal which captured the pellet and deadened the sound. After too many center hits it was easy to cut out the shot up portion of the clay and re-position. This stuff needs to be dense and it is available at air gun suppliers but far less costly and just as effective,
https://www.homedepot.com/p/Gardner-Bender-1-lb-Plug-Duct-Seal-Compound-DS-110/100212441
I shot into a steel box about 10"x10" backed with a clay like material called duct seal which captured the pellet and deadened the sound. After too many center hits it was easy to cut out the shot up portion of the clay and re-position. This stuff needs to be dense and it is available at air gun suppliers but far less costly and just as effective,
https://www.homedepot.com/p/Gardner-Bender-1-lb-Plug-Duct-Seal-Compound-DS-110/100212441
