Re: Automatic mode
It depends on your state. There isn't a federal law against it. There are certain restrictions though, which I'll try to explain below.
There is a federal law against possession by civilians (non LEO, non dealer) of machine guns that were not already owned by civilians prior to May 19, 1986.
There is also a $200 tax on the transfer of any machine gun, and you get a "tax stamp" from the US Department of Treasury (eventually) after you file the appropriate paperwork and get that stamp back. Possession without the tax stamp is a federal crime.
(Don't take this as legal advice; I'm just explaining it as I understand it.)
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