[throwaway0a5e]

The weapons they used theoretically could have been legal had they had the provenance to make them legal.

Converting your own semi-auto to full auto is in now way legal then nor now.

Full auto guns are basically a rich man's toy and have been since well before new ones were outlawed in 1986. The practicality mostly isn't there for criminal use and they're real expensive to feed.

[EGreg]

What happened in 1986 and are you saying it’s legal to own an automatic weapon if it was manufactured that long ago? Do you have to register it? And people can modify them and upgrade them?

[azernik]

See https://news.ycombinator.com/item?id=25514950

tl;dr - you don't "have to register" them, because by definition the ones that are legal are already registered. In '86 they stopped allowing new registrations for automatic weapons, but existing registered weapons were grandfathered in. US gun law considers the lower receiver to bear the "identity" of the weapon, so some pretty extensive modification can occur while it technically stays "the same gun" in a Ship of Theseus fashion.