Stopping the Iron Pipeline Act of 2021 This bill establishes requirements for incorporating passive identification capability into firearms sold in the United States. The term passive identification capability means technology that (1) enables identification by a mobile or fixed reading device, and (2) does not transmit an electronic monitoring or tracking signal. Additionally, the bill requires a gun owner to report a lost or stolen firearm to local law enforcement authorities within 48 hours of discovery. A gun owner who fails to report a lost or stolen firearm is subject to penalties—a fine, a prison term of up to one year, or both.