Answer: Automatic weapons that have not been taxed.
That is much too simplistic. There are a bewildering array of federal firearm regulations, some so deceptively worded that I feel certain there are many thousands of firearm owners committing felonies without realizing it. For example, there could be two identical looking rifles, one perfectly legal, and the other illegal (a felony offense), with the only obvious difference being the serial number. Our lawmakers have effectively created a whole new class of felon: felony exercising of the constituional right to keep and bear arms. The neat little trick about this type of felony is, if you are a felon in one aspect (knowingly or unknowingly), you forever lose the privilege to exercise your 2nd Amendment Rights in any manner. One other point: according to the federal and many state governments, there is currently no right to keep and bear arms -- only a privilege.
They were written in an obfuscatory manner specifically for that purpose, I think...