Actually it's NOT full auto but you'd never know it. It's a new, easy to install, butt-stock called 'SlideFire' for Ar-15 that does controlled bump firing. (around $350) It's apparently declared legal by the ATF. Since the trigger needs to be pulled each time by the user it technically is not full auto. However, it will empty a 30 round mag in about 3 or 4 seconds. Look at the videos you'll be amazed how it fires just like a fully auto machine gun. Gunblast does a positive review and shows how it works.
I can't help but wonder how firing range management will react when you light this thing up. You might want to warn them first as it will quickly attract a crowd.
Check it out..