Which M internals are legal for AR?

[Armament]

so who do you think will win in a court case. a letter from the ATF or the Constitution of the United States of America

This has been beat to death over at arf.com

They know better than you or I do.

section 5845(b)
For example: Thus, an AR-15 rifle possessed with separate M-16 machinegun
components can meet the definition of a machinegun, if the rifle
shoots automatically when the components are installed.

http://www.ar15.com/forums/topic.html?b=1&f=6&t=147440&page=1

a letter from the ATF or the Constitution of the United States of America

The BATFE, who regulate them I would think.