The Feds really lose their sense of humor when it comes to automatic firearms. If you are already on their radar they will go after you for mechanical failures that cause a firearm got go full auto.
http://www.americanchronicle.com/articles/view/53129
A local outdoor range has an annual machine gun shoot every fall where you can go rent time on a variety of machine-guns. It's a ton of fun but it sure can get expensive. A couple of years ago they even had a mini-gun out there. No way I could afford any time on that bad boy but it was fun to watch those that could.