When bores get older, they can be a real [beeep] to get clean, especially factory chrome moly barrels.
Using a plastic or bronze bristle brush, saturate the brush with JB and brush 25 strokes re-applying the JB every 8 or so strokes.
Then push 6-/ patches soaked with your favorite bore solvent, dry out with dry patches soaked with ligher fluid.
Next use a really good copper solvent on the bore to make sure that you have got out all the copper.
Next, measure the seating depth. When you get to the point where the 55g flat base spitzer will not touch the lands and the bullet be in the case, you know the barrel is on it's last leg.
I when the leade gets real long, I will go to a Sierra 55g Semi point (which is as accurate as berger match bullets) or the 63g Sierra Semi point. These bullets have more bearing surface and you may be able to get to the lands with them. A bullet can only jump so far before it gets started crooked. I can only guess that the bullet that has more bearing surface helps in a rough bore.
Older bores need cleaning more often and very thuroughly. As their accuracy becomes hit and miss, it is time to think of sending it back to the factory for a new barrel. I think that ruger will install a new stainless barrel on the rifle for approximately $180, and I would insist on one of their stainless barrels. Ruger's new Stainless barrels are really fantastic barrels on the dozen or so that I have seen in a variety of calibers, and they clean up as easily as my custom Hart, Lilja, Pac Nor, and Shilen barrels.
Good luck!