15 U.S.C. § 1244: Exceptions

any common carrier or contract carrier, with respect to any switchblade knife shipped, transported, or delivered for shipment in interstate commerce in the ordinary course of business; the manufacture, sale, transportation, distribution, possession, or introduction into interstate commerce, of switchblade knives pursuant to contract with the Armed Forces; the Armed Forces or any member or employee thereof acting in the performance of his duty; the possession, and transportation upon his person, of any switchblade knife with a blade three inches or less in length by any individual who has only one arm; or a knife that contains a spring, detent, or other mechanism designed to create a bias toward closure of the blade and that requires exertion applied to the blade by hand, wrist, or arm to overcome the bias toward closure to assist in opening the knife.