So you want to turn your AR-15 into a Gatling gun? GatCrank has you covered. It is an AR-15 trigger actuator that can fire 3 shots per turn. It works with standard AR-15 lower receivers with standard-shaped triggers. This video from Miculek shows how you can fire almost 700 rounds per minute with an AR-15:
More information is available here. What do you think? Have you tried something like this?