I have a Ruger SR 556 and I absolutely love it. However, the carbon build up on the gas regulator and piston is very hard to remove. I have tried M-Pro 7, Hoppe's 9, Montana Extreme Copper Killer, Blue Wonder Gun Gleaner, Break Free CLP, and even some spray-on brake cleaner. I used a good stainless steel brush and it is still too much work with any of them. What works for you?