Hard to know really how much copper is removed by soaking. I purchased a case of Iraqi made 303 British ball ammunition and the stuff had cupro nickel bullets. They fouled something awful. Unless you have ever seen cupro nickel fouling, you have no idea how bad cupro nickle jackets were. The stuff leaves a lumpy fouling and it tooks weeks of soaking with Sweets to reduce the lumps but I never was able to totally remove the things. After that, I dipped the bullets of that Iraqi ball in grease and that absolutely prevented further cupronickel fouling. But, I was really disappointed with the rate and amount of copper removal with all of the so called copper solvents on the market. Yes, they will turn a patch black, or blue, but that does not mean a lot of copper is being removed. Heck, it could be food coloring for all I know, something there to make me feel good.
What does remove fouling is J.B Bore paste, and I only use that stuff sparingly. It is an abrasive and I don't care how many times the maker claims the stuff won't hurt the bore, I can't see how it does not. So, I use it once in a while, but it does remove copper fouling.