Only two pistons out there i'd actually trust to work. HK, which is insanely expensive (and still has some issues, speaking to friends issued them in Norway), and LWRC.
Piston ARs don't do anything better than an issued M4 that's lubed and even halfway taken care of. They also move around more (accuracy issues) and depending on the manufacturer can get wear in odd places.
BCM's EAG model is out of the box, a good all-around rifle. Without having further specifications on an exact role, it's pretty close to what I'd build. My personal rifle has a few quirks, but it's pretty close to an issued M4 with some better components.
"And Shepherds we shall be. For thee, my Lord for thee. Power hath descended forth from thy hand, that our feet may swiftly carry out thy command. So we shall flow a river forth to thee and teeming with souls shall it ever be. In nomine Patris, et fili, et Spiritus Sancti. Amen."