M16 carrier is no problem.
1. There's an BATFE letter that says so.
2. My Colt came with one from the factory. If it illegal to have one, then the FFL couldn't transfer it to me.
3. Regardless of all of the above, the M16 carrier, on its own, does not make the weapon a machinegun. Not even if other parts fail. Yeah, it will cost you a fortune to debunk the prosecution's nonsense in court, and it is dangerous to count on the intelligence of a jury ...but the law is the law - M16 carrier does not make an AR-15 into a machinegun.
If it did, the BATFE would consider the carrier itself a machinegun. That's how they treat all other instances. Then again, don't count on BATFE consistency either.
Bottom line: Don't sweat it. Not because of what I've said, but because if the BATFE is out to get you - they'll get you. It's been PROVED that they deliberately modify weapons to make them illegal to get convictions. With thugs like them hiding behind a badge, you have nothing to worry about from an M16 carrier. Nothing can stop them from removing your disconnecter or shortening your barrel. Think that flash hider is welded on well enough? Even to the BATFE's specs? They will take a 3 foot pipe wrench to it. Or they'll drill, grind or modify the gun using power tools and machine shop tools and claim it was "readily" convertible to full-auto. They'll find the softest primers on Earth, load ammo and call the slam-fires "full auto" ....plenty of 100% proved, real world examples. None of what I say is made up.
If you're in their sights. You lose. "Always Think Forfeiture" is their own self-admitted motto. They'll take away what you have, and then good luck owning anything to put up as collateral for a defense. Then good luck getting a competent public defender. And if you are fortunate to actually win in court against the odds ...well, your life is destroyed. You will own nothing, your savings will be gone. You'll be in massive debt.
The bright side is, you'll still be alive.