Binary triggers are a drop in mod, as are rotary triggers that fire as long as you turn the crank. Forced reset triggers are another loophole that's become popular, they physically reset the trigger so that as long as you pull with the right amount of force, not too much not too little, the gun will continue to cycle. Its also not hard to adapt an ar-15 to have an autosear. They mostly use the same trigger configuration as a full auto assault rifle, just with the auto-sear and selector switch missing. Its as simple as drilling 2 holes and then adding the 2 parts and a spring.