So I just ordered my CMMG piston upper. I plan on building an M4 knock off with it on my current CMMG lower. Anyone have advice on piston driven AR's to impart to a noob? I plan on swapping the plastic handguard for a quad rail of some kind, but I need to get it in my hands and see what will fit first. I really dig the look of the Vltor CASV rail system, but I'm not sure it will clear the piston setup. Not to mention it's not cheap.