I brush on some Kroil and let it sit for a while, then scrub with a brass brush. Depending on how much fouling comes off, I might repeat the process a 2nd time. This is usually enough to get about 90% of it off. The bluing on my revolvers is not compromised by doing this, so I pretty much quit at this point and live with whatever the cylinder face looks like. A note about using Kroil - it gives off a strong odor, do this type of cleaning outdoors.