OK so maybe what you have is a dummy gun cobbled together from parts, except you said you shot it so that makes it a firearm. You need to find out what you have exactly.
And for anyone else reading this who MIGHT have an illegal unregistered machine gun: Guys, DO NOT TALK TO A LEO. DO NOT TALK TO YOUR BEST BUDDY. DO NOT TALK TO THE GUY SITTING NEXT TO YOU ON THE BARSTOOL. RUN DO NOT WALK TO A GOOD ATTORNEY. You have an illegal weapon and it is a very serious offense. BATFE does not take these things lightly. An attorney can talk to BATFE and if necessary arrange to surrender the weapon to them. He cannot and will not divulge any information about who his client is, not how/where he obtained the weapon nor anything else. That will be the best couple of hundred bucks you ever spent.