Have you tried Montana Extreme 50BMG? Very strong stuff. Also, how about JB bore paste? Then there's Wipeout. I've never had a bore not clean up well after using either 50BMG or Wipeout.

A little copper in the barrel won't hurt much of anything. But a lot can and usually does effect accuracy in some way.

BTW, as far as I understand, GM TEC is a carbon remover and does little or nothing for copper.

Try this site for some info on bore cleaning. You'll have to do some searching around to find it, but there's some great info there: www.6mmbr.com