In recognition of a century-old practice, the U.S. Supreme Court in Grable & Sons Metal Products v. Darue Engineering & Manufacturing, 545 U.S. 308 (2005), applied the "substantial federal question doctrine" to uphold the removal to federal court of a case that absent application of the doctrine would not have been subject to removal. As the Supreme Court explained, the doctrine "captures the commonsense notion that a federal court ought to be able to hear claims recognized under state law that nonetheless turn on substantial questions of federal law." The Supreme Court has since made clear, however, that the doctrine should be applied sparingly, and should lead to federal jurisdiction only over a "special and small category" of cases. See Empire Healthchoice Assurance v. McVeigh, 547 U.S. 677, 699 (2006).

In New York v. Arm or Ally, 2022 WL 17496413 (S.D.N.Y. Dec. 8, 2022), Southern District Judge Jesse M. Furman recently applied the substantial federal question doctrine to retain federal jurisdiction over a lawsuit by the state of New York, in which the state asserted a variety of state-law claims (but no federal claims) against 10 defendants (the defendants) based on their alleged manufacture or sale of "unfinished" firearm frames and receivers. A "frame" is the core part of a handgun pistol, and a "receiver" is the core part of a rifle, shotgun or other long gun. According to the state's complaint, the process of converting the unfinished frames and receivers sold by the defendants into functional firearms is "ridiculously easy." Such frames and receivers are commonly known as "ghost guns" because they are not stamped with serial numbers or otherwise registered and, therefore, are untraceable when recovered by law enforcement in connection with a crime.