From the replies given, I see there is no single consensus. Even if you know the owner, you should not necessarily assume that he is the victim and wack the other guy with the tire tool or whatever.
ATeaM had something of a bizarre response. The owner and another are fighting in the store, so he would grabbed the shotgun to go looking for a getaway car. Of course, then he would steel the gun.
C.R. Sam's advice about making the pawnbroker aware you are there and asking if he needed assistance is a good idea, but still could problematic.
vertigo7, I don't see why you didn't make a phone call. You identified the pawnbroker in a scuffle on the floor with a shotgun nearby and you decided to wait to call 911 until you had a better idea of what was going on? You have to know that the pawnbroker and some other guy on the floor behind the counter engaged in conflict is not normal business proceedings. At that point, you definitely should have called 911. At worst, the cops respond to a misidentified situation. At best, they arrive quickly to help bring the situation under control.
I really don't believe the pawnbroker's story that the two men were just rough-housing. There has to be something else going on beyond that. I think FPrice has the most prudent answer.