Re: HK MR556
If you wanted to go that route, it would probably be best to go get one of the actual HK 416 uppers that can be had (usually they are in the NFA length range but that doesn't matter with a MG) and put it on a registered F/A lower (non HK). Use the new recoil buffer / spring setup that comes with the 416 and you are basically there.
The MR556 upper and lower are both designed not to accept F/A parts / operation without a fair bit of metalwork.
I saw one for sale where someone had taken a registered F/A lower and cut in the HK markings so it made it look just about exactly like the military version of the HK 416, just if you looked really close, you could see the original engravings in black on the receiver as well as the new HK copied ones (which were in red / white)