Wouldn't be the first time. The Lautenberg Amendment is the poster child for ex-post facto laws.

But it's OK because:
  • It's for the children
  • It will reduce gun violence
  • It will get weapons of war off the streets
  • It will keep hunters from using 30-round assault clips for shooting deer
  • Liberals hate the Constitution anyway
  • Obama shoots a shotgun, why can't you?
  • The cops, who have automatic rifles and armored vehicles are outgunned
  • You'll do it and you'll like it