I shot at the OKC gun club rifle bay once that has the concrete benches. The top skin is sheet metal and they sprayed insulation underneath. Above each bench there is no insulation in a circle. I was told that was because of brakes jetting the gas upwards and blowing it out. Some jet the gas sideways and some up evidently although I've never seen one that jets the gas up other than small ports to mitigate muzzle rise. Brakes are very effective in what they are designed for, but detrimental to maintaining normal hearing.