i have acquired many rifles over the years that have been "Seasoned" . the thing that is common with them is the cleaning patch with say Alcohol, ballistol, or any type of solvent, will keep showing black rifling streaks until your shoulder falls off.
one i got not long ago, i bore scoped. the lands were nice and shiny. the grooves were black as if coated with Teflon. i started with a brass brush and solvent. followed with patches soaked with solvent. followed with patches soaked with Alcohol. then brass brush. repeated this until those patches came out without the skid marks.
bore scoped it and had shiny metal in the grooves. loaded the same, but shrunk the group a bunch.
only took 110 patches and a bar of soap. the soap was to clean out the fouling in my mouth! wife takes offense when i challenge the parental relationship of an inanimate object.
i have a tube of bore butter. think i bought it to use with a 9mm cal Spanish pos rifle i traded some Plymouth hub caps for in 1970. 99.5% still in the tube. smells good though.