No, I don't think that S & W makes a specialized .38 cylinder for a 686.

I'm having a little problem understanding why removing the carbon left over from .38s in your 686 is such a big deal. How often do you clean? I shoot .38s from my Models 27 and 19 all of the time without any carbon buildup problems. I do clean after every trip to the range. I brush my chambers with a .40 cal. brush (sometimes I use a .45 cal brush if I think I need a bit extra friction). Usually a 1/2 dozen passes will break all of the carbon free.