The M14 receiver is a machine gun. Though many were in use as semi-auto only (for practical reasons), it was only because the Army either did not install, or instructed armorers to remove, the few linking parts which would indeed allow it to fire full auto. If you look at your M14 receiver (out of the stock) you will see a metal tab sticking down from the right-hand (as viewed from the rear) side. That is the key element. If there is not, you have one of those to which Chuck refers, which was ground off. I do not know of the legalities there, though I am sure there would be a snowstorm of paperwork associated with it.
Your stock may even have a cutout at that point. There was a key shaped switch there, along with one or two internal parts and a bar linking to the operating rod. The issue is that, unlike rifles like the AR15/16, for example, you do not have to make modifications to the rifle to get auto fire. If you added a full M16 trigger pack to an AR15, you would not get it to fire full auto without adding a sear. But, an M14 only needs reinstallation of readily available parts to go back to full auto. BATF policy is "once a machine gun, always a machine gun." And, though I hate to say it, given the parts issues, they are correct.