Why do you want a piston?
Tests have shown that a piston system on an AR is a solution to a problem that doesnt exsist. (Pat Rodgers running 31,000 rds through a BCM upper without cleaning and zero malfunctions)
Save the weight, save the $$$$$, and carrier tilt...and go get a quality DI AR.
They run ALOT Cleaner and Cooler is the main reason I run them.
My piston gun balances better with the weight, cost was comparable, carrier tilt has never been an issue on any I have used. LMT and Sigs are quality