A family member in MD wants an M4 (16" with 2" FH) copy and I agreed to do the build. What's the best way to proceed? Should I:

Put it together here, then next time I'm in MD bring it out and do the transfer then?
Or, have everything shipped to MD and have him do the FFL stuff and then put it together when I get there?
Also, the way I understand it you can only have HBAR's out there?
If the barrel is heavy under the hand guards but has the M4 notch is it still a HBAR?
Lastly, are both receivers controlled items or just the lower one?

Thanks in advance for your help.

I'd have him buy the lower through an FFL.  If he doesn't know of one, some of us can point him to a reasonable one.  I'd send the other parts here and do the build in front of him so he learns.

Doesn't have to be an HBAR here.

HBARs are sold 'over the counter' following applicable federal rules.