If you have time, skim the first page of the link inpost 16. The weapon had a filed sear and other mods to make it full-auto. The owner seems to be claiming that the guy he loaned it to (for months) must have made the mods.
Many of the parts used in the semi-auto version can easily, and legally, be swapped with those in the full-auto. There are several that are key to getting them to function full auto. You need 'em all to get it to function as a full auto. Any idiot can file down the disconnector and do mag dumps all day long. This is a MALFUNCTION. Not "full auto"/"select fire". You can do the same exact thing with ANY semi-auto firearm.
I put an ambi-safety on one of my builds. The Selector will go about half-way back to where the markings for a Full Auto position would be. Pull the trigger, nothing happens. Now take some ham-handed Feebie trying to prove a case to take another manufacturer out of business... and who knows what it'd do.
Watch The Gang to see what I mean.