I it has kind of burned black deposit, you may also want to use mechanical brush with metal polish :

http://www.brownells.com/.aspx/pid=6...NSMITH_BRUSHES

Brass brush cleans pretty effectively but does not scratch stainless cylinder. Take few oversize (for example, .40 or .44 brush for .357 mag) bore brushes for chambers.

I used to clean my silhouette revolvers cylinder face with steel brush, but I do not necessarily recommend it...