I'll kick in even more reasons if I may.
1) Full auto is damned hard to control. It takes practice. Where you gonna practice? Every time you let rip you gotta worry about the weapon detonating on you, or the cops showing up.
2) Full auto is expensive man! It don't take long to go thru a case of 1000 at full auto.
3) Full auto is hard on weapons. Yes, they were designed to be fired full auto (with full auto parts installed you understand), but that doesn't mean it's necesarily good for them.
4) By converting your semi to full auto, you make the jobs of the Antis and the Bradys that much easier.
5) If you get caught....No, WHEN you get caught, as ZZ said, you will spend a great deal of time incarcerated in a Federal Correctional Facility. For the rest of your life, you are a FELON which means you will never, ever, ever be able to own or even be in the same room as a gun. Period.
IT'S JUST NOT WORTH IT.
If you must waste ammo, learn to bump. Bumping is just about as accurate as full auto, and it's legal baby!