I've got a Remington 700, 25-06 that coppered badly. Found a product at a gun show that has enough ammonia to clear out a room, Montana Extreme. As the old TV commercial said "Try it you'll like it!"
Another option, fire lap the barrel, problem will be solved.
Jim

Um... Barnes CR-10, anyone?

Russ

Hey, even with WO it still took NUMEROUS treatments to get the copper out. jorge

I think if you try the same thing with a standard bullet you will see the copper streaking as well, winchesters of late don't have glass smooth bores. Remember copper in the bore is not copper FOULED, a bore is fouled when the accuracy begins to deteriorate, before that it is just being used.