Depends on who you talk to. I seem to remember a court case that said it was legal to build your own machine gun. But you can never take it out of State nor could you ever sell it. Also, the Feds may very well charge you in any case.
I have so far been unable to get the full text of the NFA in any readable layout - what I have found is a mess of provisos, addenda, quidproquos... lotsa references but no full consecutive text... beginning to wonder if there ever really was such a thing as the NFA