Go buy some Castrol SuperClean degreaser and a $50 ultrasonic jewelry cleaner.
Mix the SuperClean with 1/2 water (it is water soluble) and leave it in the ultrasonic cleaner for about an hour.
I have gotten ~~BLACK~~ AK pistons to look brand new with this. The Castrol SuperClean is biodegradable and water soluble and you can put it down the sink with no guilt.
The ultrasonic cleaner I have is a cheap type with a bath just big enough to put a disassembled AR bolt + carrier in it. If you do this once every few hundred rounds, you'll be amazed.
Here is the cleaner I refer to:
You'll never worry about carbon buildup again.