I don't personaly use any sort of abrasive on a gas piston, or inside a gas cylinder, because doing so can/will eventualy efect their dimentions.
Soaking a gas piston over night in a good solvent and then wiping with a rag has worked fine for me. If you find it nessesary to use a brass cleaning brush inside the cylinder, go gently and let the solvent do the work. A brass brush is relatively soft, but the loose carbon that you'll be rubbing-around in there aint....if you get my drift.
And bear in mind that it's easy to mistake burn-marks for carbon. Burn marks are merely discoloration, and don't hurt a thing.
Hope this helps.
DGW
