I had an 8X57 rifle with a bore like that. Could not get it clean. Read somewhere that long, continued, use of ball powder could do that. Got really aggressive, and took after it with coarse steel wool wrapped around an old bore brush. Soaked it with solvent, and went at it twenty strokes at a time, then change the steel wool for new, as it was coming out quite dark.
It was quite an amazing transformation. The bore was bright in less than 100 strokes, and the rifle went from a mediocre shooter to a very satisfactory hunting accuracy. The more it was shot after that, the brighter the bore became.
You might want to try the cave-man treatment....
Ted
"When you ain't got nut'n .. you got nut'n to lose"
Have saved a few over the years, a couple of 'em were rusted almost shut, with those cave-man excavation methods.
Alternating .. brake cleaner, Evaporust, JB bore paste, Kroil, CR10, steel wool. ... repeat till the grooves re-appear.
Like peeling off layers of onion skin.
One rancid-bored old .303, it got to outright gleaming after a few hundred fax paper patched cast bullets went through. Seems that fax paper has some abrasive polishing clay in it.