I've been shooting and reloading 204 rugers for past year, never any problems unill today my good shooting buddy went to his range and shot about 30 rounds of 204. About 28 of the 30 rounds came out with black on the case from 1/4" from end of case to the shoulder. He has loaded longer than myself, but either of us has ever seen this happen before. He shot one 3/8" group at 100yds , all seem fine for accuracy . Can anyone give some advise as what would cause the black cases?