(It's not related to the incident, but I do also wonder what that blue crap is in front of the where the rifling ends)
Dissolved copper residue?
I'm not expert on Smiths, but it looks like there is a real deep crown or perhaps abarrel shroud. Perhaps a little fluid is pushed onto the ledge from moist patches, and then not picked up from the corner by dry patches. Or, if there is a shroud, perhaps a little fluid is getting into the seam between barrel and shroud, and then leaving the copper residue when it dries.
Maybe take a Q-tip to it after cleaning the bore.