Barrett M82 50 caliber rifle fired by several people at an indoor range. The gun was set up in the smaller side of the range, which is about 6 lanes wide, making the concussion even worse. Two speakers fell off the wall while it was being fired. It was so dirty it would not feed rounds from the mag, so each round was placed into the chamber and the bolt manually closed. We were told to aim low, because the backstop got thinner at the top.