I have a number of old Marlins and bore conditions very from pristine to awful. All have shown some degree of acceptable accuracy. I've found even bores that that look really bad get better with shooting, cleaning, shooting, cleaning, and more shooting. More crud just keeps coming out.
In this video is the rifle with the poorest bore of all or my old Marlins. I had already decided to reline the bore so felt I had nothing to loose by firel lapping the bore. I didn't see any real difference in the bore from the exercise. Apparently, it would take many more rounds to effectively polish the bore.
I wouldn't recommend that anyone try this unless you're as crazy as I am.