[4][6] Early examples of pen guns were pinfired, but modern designs are rim or centerfire.
[1][7] In the United States, pen guns that can fire bullet or shot cartridges and do not require a reconfiguration to fire (e.g., folding to the shape of a pistol) are federally regulated as an Any Other Weapon (Title II).
They require registration under the National Firearms Act and a tax in the amount of five dollars is levied.
[7][8] According to the FBI, pen guns were widely used for self-defense in the 20th century.
[9] Media related to Pen guns at Wikimedia Commons