A buddy of mine is on the last couple weeks before his transfer goes through on a select fire M16 lower receiver. I’ve been talking to him about selecting an upper receiver, and a question came up. If he decides to go with say a 10.5" upper, does he need to go through the process of registering the lower as an SBR as well, including engraving the lower + another $200 tax?
No he will not need to register the MG as an SBR. A MG can have any length barrel on it.
On a slightly different note…If you have a standard AR15 lower (not registered as an NFA item), you can install a RLL or RDIAS and use a shorty upper. If the RLL or RDIAS is removed from the receiver, you would have to remove the shorty upper too.
What exactly is RLL and RDIAS? I’ve tried to search it but I can’t find a good explanation.
Registered Lighting Link and a Registered drop in auto sear which will allow a semi auto lower to be turned into a full auto but only the RLL and RDIAS are the registered part when it is removed the weapon then become a semi auto again with the restrictions of the 16 inch barrel
RLL = Registered Lightning Link
RDIAS = Registered Drop-In Auto Sear
Either one will allow you to legally convert most AR15 lower receivers to full-auto since the parts themselves are considered to be the “machinegun”. Last time I checked, the going price was well over $10k for either one.
yelp $13k was the price of the last one I saw for sale.
The RDIAS are much higher than the RLL. It has been a few months since I’ve priced them. Last I looked the RDIAS was about $12k and the RLL was about $7.5k.
The downside to the RLL, IMO, is that it is virtually impossible to get it working with a .22lr kit.
But everyone’s in agreement to the original question? (It’s not that I don’t believe you DrBaker, just want to be sure. 
)
If the lower is a registered full auto lower you can put any upper on it. The machine gun trumps the sbr.
DrBaker is correct.