I use MPro-7 with a brass brush or well-used copper bore brush (careful not to scuff with the steel end). I have a metal polishing cloth, but am worried that I'm taking off some of the actual cylinder (yes mere millionths of an inch, but still).

For blued guns I still use Mpro-7 but rub with a regular cotton cloth. Doesn't take off nearly as much, but no way am I scrubbing with a copper brush!