[Bindernut]

Yup, unless your bore is polished perfectly smooth it'll always hold a bit of copper fouling in the pores of the metal.

I do about like Josh. I'll clean until the real deep blue stuff is gone.
After I'm satisfied, I clean out the ammonia-base cleaner (I like Sweets7.62 or Barnes' CR-10) with Kroil. Final step is to oil with a heavier oil, like RemOil.
Even with a plastic or steel jag and using only nylon brushes you'll still see a blue tinge regardless of how long you scrub.