That's incorrect, they are legal federally, but some states have made them illegal.
In order to purchase an automatic weapon, you need to submit a form to the FBI, provide fingerprints, have your local police chief sign-off (or create an LLC) and pay a $200 tax.
It's a very thorough background check.
Not many people buy them since the cheapest weapons cost $10,000 and go up to $50,000. This is due to the limited supply since civilians can only buy automatic weapons that were built before 1986.
It is about $500-$1000 USD per year. You need a secure place to hold your arms, keep extremely well maintained records of anything you buy, and if you stop paying your SOT any post-86 things you have manufactured must be destroyed.
If think you might also have to file a form 1 for everything you manufacture and that takes 1-5 months even stretching out to a year in some cases.
This also comes with a round of your prints being sent to the FBI, ATF, and I'm assuming a bunch of other three letter acronyms that you don't ever want to meet.
But, if you can do that, get a drill press, and make friends at your local PD you can make some good money on the side by building post-86 dealer samples and handling transfers for your local PD.
Believe it or not, but by my current understanding factories cannot ship guns directly to police offices, they must still be handled through an FFL.
I found yesterday that full automatic assault rifles are not legal in the US but that's the first shit any idiot brings up here when they want to paint Americans as "crazy".
Don't get more wrong, as things progress in Europe, we're eventually going to need those "crazy" American weapons.