Speaking of barrel cleaning, been experimenting w/ slugs. Talk about dirty. Upon removing shroud on Impact & Cricket.there were " chunks" of lead on and around the crown.
I remember when shooting cast lead bullets out of a 357 revolver that I had a problem with lead building up.
Slugs would be similar to cast bullets. We shot them at higher velocity and I’m thinking that might be a factor in this.
Now the next question why not pellets. I suspect at high velocity they might lead up as well.
Just a guess
Probably just slug fecal-matter. Either clean the bore regularly, or adjust your slug's diet to include more fiber.