No registration required to build an upper to switch out in an AR platform ...just be sure to change the recoil spring and buffer, too.
I actually built the whole project and leave it intact. It’s got an 18 in bbl with mid length gas and it was the first AR I built with Midwest Industries full float fore end which I really like. I put a VLTOR stock with a nice pistol grip/trigger guard set up all in FDE. It’s very light, deadly accurate and so far very reliable.
I bought about 5 boxes of ammo; some Hornady and some PPU, and then Hornady dies, brass and projectiles. So far I’m only using 120-123 gr bullets and the trajectory out to 400 yards is impressive for not being a 3000+ fps round. I still have yet to CHRONY the stuff.
One thing I don’t like, not a big deal but, it dents the brass mid case I think against the ejection port.