Registered fullauto trumps SBR. If you have a M16 you can put any upper you want on it.
If you want to suppress it you have to buy a tax stamp for your suppressor.
Also, don't have a AR-15 and a M16 and to many uppers shorter than 16 inches. YOu start treading in the constructive possession territory then.
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.