Just a suggestion before shooting it.
I used to scrub the bore on rifles whihc looked worse then your photo. What I used was regular Hoppe's #9 on medium steel wool.
I wrapped the steel wool around a smaller bore brush, like 25 cal, and dipped the whole thing in Hoppe's then vigorously scrubbed the bore, soaking the steel wool repeatedly.
Things will get very dirty very fast so don't do this on the kitchen table for risk of invoking the wrath of wife. After running this through for at least four or five runs, put a few clean patches through the bore...they will not look clean though may improve. Avoid using an abrasive polishing compound, the steel wool and Hoppe's scrubs out the crud, though it is very messy.
Cheers