Sandman said it best. Almost each weapons system / platform has its own rules for compliance wrt. NFA. Just because one build is "legal" for one platform doesn't make it legal for another. 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. This has happened even with M16 F/A parts kits. Now most reputable dealers won't sell you these goodies without proof that you own a registered firearm, as the BATFE is also holding them just as accountable as the ignorant people that get said parts for the "koolness" factor.
If you search the boards on here, you will find lots of info on this from good folks. There is thread on here about what makes a m1 vs. m2 especially from a parts kit/NFA compliance perspective. When in doubt, call the man and get them to send it to you in writing.
I and a bunch of people were recently under the "wrong" impression that non-sporting firearms were 922(r) exempt, well we were wrong. Just b/c they are registered under NFA doesn't exempt them from 922(r) when in doubt, call the BATFE, get clarification in form of a letter. If you assume, it could be a costly assumption and may cost you your freedom.