Untraceable Firearms Act of 2021 This bill establishes a federal statutory framework to regulate the manufacture, sale, offer to sell, transfer, purchase, and receipt of ghost guns (i.e., guns without serial numbers). Additionally, the bill revises the federal statutory framework regulating the manufacture, possession, sale, import, shipment, delivery, receipt, and transfer of undetectable firearms. Among the revisions, the bill defines undetectable firearm as a firearm without a major component wholly made of detectable material.