I would be very tempted to keep my mouth shut, but my problem is I already own three legal machine guns one of which is a Thompson. If I was caught I would loose all the machine guns plus a few dozen assorted rifles and handguns and spend the rest of my life in jail.
I hope I never am faced with that decision because I would be very tempted.
People that own these guns keep the stamp locked in a safe and carry a copy when they go to the range. If you are at a range and asked to show your stamp you could get into deep voo doo if a cop asks to see it and you can't produce it.
so owning one of these guns without a stamp means you are taking a risk by shooting it so the gun is something you can look at but not shoot.