Every once in a while, you come across something that makes you say "God Bless America!"
I had the chance to shoot somebody's AR with a slide fire stock on it last night. For those who haven't seen it, the stock slides along the buffer tube and has a piece that goes up along the grip and over the trigger guard. You put your finger through contacting the plastic so the tip of your finger touches the trigger. You then push forward slightly on the fore grip. The recoil keeps contacting the trigger. You can fire controlled bursts of anything you want from 2 to a full mag with sufficient accuracy. I fired controlled bursts immediately without learning any technique.
Technically it's considered bump firing, the trigger is pulled by your finger for every shot. It comes with an ATF letter showing that it's legal. http://www.slidefiresolutions.com/Product-SSAR-15.html