Here is a link to the Adams Arms piston. It is the one used in the S&W M&P 15PS. There is no gas key or staking to worry about. Having shot and cleaned DI guns, my M&P piston gun is the best I have ever owned. The only place it gets a bit dirty is the front of the handguard, but it cleans easily with a rag and Hoppes 9. By the way, the weight of the piston rod, spring and bushing is 6.5 ounces.
http://adamsarms.net/images/explodedviewdiagram.pdf
http://www.adamsarms.net/