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The soot is, as you have deduced, a low-pressure sign as the case isn't pushed out hard enough to seal off the chamber completely.
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I have to add a bit of further explanation to this, its not that the case doesn't seal against the chamber, its that the lower pressure doesn't tightly seal the case against the chamber BEFORE some of the powder gas gets on it, which leaves the soot marks.
Other than cosmetic, this is seldom a problem.