Just in case the usual copper fouling remedies do not work the Gun Digest with the article on Mobil Lubricant in the 2010 edition would be a good read. Those original cupra-nickel bullets left a fouling that had a particular affinity to the steel in the barrel. In that day the only solution was using the Mobil lubricant applied to the bullets at the time of shooting or using a very corrosive ammonia solution to clean the bores. Applying the lubricant just before shooting was trouble some and too much caused a few blown up rifles. Interesting read if you can find it pages 118 to 125.
King Carp if you happen to have one of the few rifles that had the fouling and did not get cleaned up, drastic measures may be required as the usually successful methods of removing copper fouling today may not work on nickel.