The best copper remover I have seen and seen them all, Barnes, Eliminator, wipeout etc) and the best is Montana Extreme 50 BMG.
Use a nylon brush and let it soak or 10-15 minutes. It is the probably one of the most common used by LR BR guys and they use borescopes to check results.
Second after you get it clean, buy a $28.95 David Tubbs Final Finish kit and use it. It has bullets with a varying lapping compound on them. Note, the kit calls for firing 10 of each grit, but I normally only fire 5 and it seems to work quite well.
Go to www.6mmbr.com
and read their blog, they just ran an article on this very subject.
The other problem you have, is carbon buildup in the throat and BMG will also remove that.