The "amplified" effect you refer to is just the same level of sound but it bounces back and forth from walls, floors and ceilings so rapidly that you perceive more sound. And yes, it does do more damage than outdoors.
+1 to this. When my incident happened the wall to my left was much closer than the wall to my right. As a result I have absolutely no damage to my right ear according to my hearing tests, but in my left ear I have minimal high frequency hearing loss and a moderate case of tinnitus. Considering I was using a revolver too i'm betting the blast from the cylinder gap shoots sound waves straight out left and right from those gaps, just a theory but it makes sense.