I dont Understand, you said you left it at the Pawn Shop and when you picked it up it had a (Spent Primer) on it? Well if the Primer was spent then it should have been shot and not loaded, why would you put a spent Primer on a loaded ML?
I have left ML'ers loaded from the end of Hunting Season untill the next Fall and they shot fine, but they were also cleaned and oiled befor they were loaded. A fired ML that was reloaded and then left for a long time may have corrosion in the barrel, I have seen many ruined this way.
Good Luck!