function or not, if that 80% finished lower has a serial number on it, it is legally a firearm. Don't take chances, especially in NYC!!!!!
Their rules on what is legal and what is not are different from virtually everyone else's. Would be a real shame to wind up in jail (or not and still spend thousands in legal fees) for a simple gas gun.
All else being equal (and it almost never is) bigger bullets tend to work better.