For anyone with an oversized bore brush, the very minor cylinder ring left in a .357 cylinder after firing .38 Special cartridges is about a ten-fifteen second job for all six cylinder throats. It never hurts to run a brush through the cylinder throats anyway, even after firing .357 Magnum cartridges. This is just a part of routine cleaning to prevent fouling buildup.

I've never seen a ring as bad as described by some folks. If the ring gets to that point, it's a result of neglect.