Two things:
1. Did we answer the OP's question yet?
2. You'll see pigs flying around the sky before you see new MGs added to the transferable registry.
Politicians have an easy time passing laws like "March 12th will be Hank Aaron Day."
But passing a law called "$800 M16s For All Who want One" would take a years-long national political push on the order of passing Obamacare or passing the Brady Bill.
It would take a true political bloodletting and the broad public support will never be there for it, because the optics are bad. In 13 HEAVILY POPULATED states you cannot even own a transferrable MG.
The other possibility would be to have the Supreme Court declare Hughes to be unconstitutional. And there is no reason to think that they would do that. They are arguing the bump stock ban right now, but the arguments revolve around the definition of machine guns. If they decide that the bump stock is indeed converting the gun to a defacto machine gun, then the ban will stay in place.
Sometimes you see gun owners on these kind of forums complaining that "rich gun owners don't want their $30,000 guns to be worth $900" so they are somehow keeping the transferrable registry closed. That's true: a handful of gun collectors are using their Magical Mind Powerz to control 160 million voters. Personally, I control all of the eastern seaboard with my Mind Powerz. It's a lot of work.
When the Democrats manage to get all those millions of illegal aliens voting rights, which they are already talking about, this country will be officially a one-party state and for the rest of your life you'll be handing your money over to the Democrats so they can buy votes from poor people with it. So get in there and vote R or you can kiss everything you own goodbye.