The NFA was pretty much re-written in '68, and that is the law that would need to be repealed. I say repeal them all.
The only problem would be that owners of Class III firearms would see the values drop precipitously. A transferable M-16 from before '86 is currently worth about $20K. If these acts are repealed it would probably drop to about $2K in value, depending what the manufacturers would charge for a new one.
Just because I own a couple cars doesn't mean I want car prices to continue to skyrocket. In the short term, you'd still be able to trade one you own for a different one, even if prices plummeted 90%. In the long term, if those are the kind of guns you like, you WANT them to be cheaper and obtainable with fewer infringements on your rights. And philosophically, I'm sure that anyone that's into freedom enough to own a machine gun would gladly pay $18K to live in a free country. I would, and I don't even own a machine gun.
I have a NFA weapon and would gladly take the hit on the money if it was repealed. S&W 76 was supposed to be named that because that was what it cost to manufacture. Cost me $6700