How did this incident come to the attention of the BATFE?
Whoever he lent the rifle to took it to a range where it doubled, and some LE also at the range wandered over for a closer look. The rest is history.
Back in the day you could buy “drop in auto sears” for AR15 type rifles. To convert the AR15 to selective fire all you had to do was replace the hammer, trigger, disconnect, selector,and the bolt carrier, with M16 components add the auto sear which required no modification to the receiver. The ATF soon classified the “auto sear” as a machinegun which had to registered itself. Anyway a lot of AR15 types are still around with the M16 components left in the lower receiver. If you move the selector to AUTO the hammer will follow the bolt carrier forward and possibly slam fire or “double.” The definition of a machinegun in Federal law is any firearm that will fire more than one round with one pull of the trigger. With some difficulty this individual’s AR15 was made to double and he was convicted of illegally transferring a machinegun.