Calling it a "machine gun" is a bit of hyperbole

Allegedly converted to fully automatic which would be a machine gun. Not hyperbole at all if it was converted.

Full automatic guns are illegal in the US unless a special license is obtained. That license is expensive and needs annual renewal. A teenager would not have one.

This is not even close to being correct. The most common way to own a machine gun in the USA is to purchase one and pay the $200 tax for owning it. There is no renewal fee. Now, since no new machine guns have been allowed to have been made for the public since 1986 even the least expensive machine guns are many thousands of dollars.

You can get a manufacturer's license to build the things, and those licenses are good for three years and do not involve an annual renewal.