In the spirit of Greg’s posts:
“is the m4 lower the same as an ar15 lower”
Not quite. The design is identical with one difference, a small hole drilled above the safety selector hole, used to mount the auto sear, which allows full auto fire. An AR-15 does not have this hole, and cannot fire full auto without modifying it, adding a Drop-in Auto Sear or Lightning Link. Adding the parts is legal, so long as they were manufactured and registered with BATFE before the ’86 machine gun ban. The unregistered DIAS is illegal. Any other attempt to modify a semi auto to full auto is illegal. Don’t do it.
An M4, or M16, uses a different fire control group, lower receiver, and bolt carrier than an AR-15. My understanding (IANAL, use this with caution) is that possession of the M16 bold carrier is generally OK , however, possession of the fire control group parts is likely to cause the BATFE to ask a lot of difficult questions. You probably can’t get the lower receiver without it being stolen or illegally imported, either of which are going to cause similarly embarrassing questions. An M16 lower, which is functionally the same as an M4 lower, can be purchased pursuant to the NFA, so long as it was manufactured and registered prior to 1986. A good place to start is subguns.com. Be prepared to spend a lot.
This is not legal advice, before doing anything involving machine guns, you would do well to consult with a lawyer.
UPDATE: Corrected link at the top to link to the actual Q&A posts.