I agree with oldelkhunter, you need to use a dedicated copper solvent. I've used Barnes CR-10, Sweets 7.62 and Wipeout. Wipeout is supposed to be a universal do-it-all type bore cleaner, but I use it strictly for copper fouling since in my experience it doesn't do a thorough enough job on powder.

If I had to rate them I'd put them in this order:

1. Wipeout
2. Barnes CR-10
3. Sweets 7.62

I think CR-10 actually does a better job than Wipeout, but I prefer using Wipeout because it's easier to use and less noxious.