There is a video done by a Class III FFL who legally possessed a "switch" for a Glock. He took a stock Glock 17 to the range and demonstrated the swap out of the rear plate on the slide for the "switch". He then installed a large mag and turned it loose. It indeed fired full auto. Possession of the switch by a non-Class III FFL is a felony. The harsh truth is they are sold in bad parts of town for $30 to $50.
