tried shooting some wax, the first one I shot using a large pistol primer. It didn't even dent the box. The second one I used a mag large rifle, wasn't sure if the pistol would set it off, it did. Was probably 4x as powerful as the pistol o/primer.
another option for indoor shooting are the Speer plastic bullets. we used the 38/357 plastic bullets for 9mm and 380 cases powered by a primer and a cardboard box with old towels stopped them. haven't tried the rubber bullets.