It's lead residue...I always noticed the lead film after a pattern testing range session and felt it was possible some amount of moisture / trace of fouling 'could' be trapped under / in it so it had to come out.
The simplest, easiest, quickest, and even enjoyable way I found to get it all cleaned right out was at the end of the range session, I just shot a few of my snug fitting PRBs through it and the tight fitting patches dragged / wiped it right off the bore walls.