Dear Kitch:
Have you tried using a 270 Win. chamber plug (Brownell's has them), filling the bore with Shooter's Choice and letting it sit upright in a corner for about three days?
That will dissolve the copper, and not harm the bore.
I use a zip-loc bag and a rubber band over the muzzle to keep the solvent from evaporating, and to cut down on the smell.
Sincerely,
Chris Bemis
