What was the lube? Some lubes are NOT meant for fast rifle speeds or long barrels. IF the gray comes out easily then you don't have much of a problem. Not knowing WHICH alliant powder used is another ? as some are smokier than others. That gray MAY have been powder fouling OR leading. You may have got the barrel quite hot shooting 16 shots in a hurry, but I doubt that would cause leading. I am glad you took the plunge and started using cast. Now the real fun begins for you. New molds and alloys AND lube experiences are right around the corner!