Here's what I know
1. Stag is an OK brand if you just want to shoot for fun but I wouldn't trust my life to anything they make. (or anything less than a colt for that matter)
2. A piston system on an AR15 is over rated and unnecessary as original direct impingement system works just fine. And they also aren't standardized so any replacement parts on the piston have to be from the OEM.
If you are just plinking with it then just shoot it (when you can get ammo) and see what happens. If you are thinking that maybe you should have bought a better gun then check this out. http://www.m4carbine.net/showthread.php?t=7376