I had to go tear my M2 apart and I am WRONG. The receiver is the same as my M1, the trigger housing is what I should said about being machined. I am sorry for the confusion. I do apologize for my mis-statement about the receiver though.
In the case of M2 Carbines, having the FA parts kit makes you the proud owner of a machine gun according to the BATFE, regardless of your possession of a m1/m2 rifle or parts.
That isn't what the ATF told me in Omaha when I approached them in 1999 with my Inland M1. It had several of the M2 parts in it and they told me as long as your rifle/gun isn't machined or started life as a full auto weapon the parts do not make it a machine gun as it can't be fired in fully automatic with out further modification. Now maybe he was blowing smoke? I don't know that is just what he told me when I received my M1 that had the FA sear, hammer, disconnector and plunger and the round bolt.
I would be interested in buying his parts from him though for my registered M2