It looks to me like it is shaving lead. Either a slight misalignment of cylinder or a timing issue. Also, how is the forcing cone of the barrel, is there a beveled lead at the breach end? I have seen some that had almost no bevel at all which adds to the problem. This may be a case for return to S&W. I'd call them and see what they say.

It can be cleaned off with soaking in a good solution and brushing it off, but that doesn't address the problem.

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H Richard
SWCA1967 SWHF244