Sorry if this has been asked and answered before.

Could a current M4 have the FA fire control trigger group replaced with the semi-auto ones, then delisted from the NFA registry (or maybe listed as only an SBR at that point)?

No - the receiver is legally a machine gun, regardless of what fire control components are installed.

The closest you'll be able to get is an M4 Carbine labeled Colt semi-auto rifle.  Unfortunately there are other differences, since on their semi-auto rifles Colt mills off the selector stops and engraves "Safe/Fire" rather than Safe/Semi (/Auto).

-bob

Per the ATF, once it is a machine gun, it is always a machine gun. That said, semi auto lower receivers and lower parts kits are cheap these days.



I have a retro-ish AR-15 made from a demilled M16a1 parts kit. It is not in any way a machine gun, although most of it was in a previous life.

+1 for the above comments.

FWIW, i have seen some clones that look identical to Colt M4/M16s. Ask around in the MK18 thread, those guys are great at what they do. One person took a 80% lower and had it milled and roll marked just like a Colt, was pretty stunning. Just remember not to drill the hole for the sear pin, thats not legal, or if you do make sure its intentionally misaligned, the wrong size, etc...

Good info, thanks guys.  I already have a Colt LE69320, but wasn't sure about changing the FCG and delisting (I've seen that done on SBR lowers).