Lawful Purpose and Self Defense Act This bill modifies the definition of "armor piercing ammunition" for purposes of federal firearms provisions to: (1) include a projectile that is designed and intended by the manufacturer or importer for use in a handgun; (2) exclude a projectile that is primarily intended by the manufacturer or importer to be used in a rifle or shotgun and a handgun projectile that is designed for hunting, recreational, or competitive shooting. This bill repeals a prohibition on assembling from imported parts a semiautomatic rifle or shotgun that is identical to one prohibited from importation as not being suitable for or readily adaptable to sporting purposes. This bill repeals the condition that the Attorney General must approve, as suitable for sporting purposes, the shipment of firearms or ammunition to members of the Armed Forces. The following categories of firearms or ammunition may be authorized for importation into the United States by the Attorney General: (1) ammunition that is not armor piercing; (2) a firearm or ammunition that is being brought in for the use of a federal, state, or local government agency; and (3) a firearm or ammunition that is being imported for the purpose of exportation. The definition of "destructive device" for purposes of federal firearms provisions excludes: (1) a shotgun or shotgun shell which the Department of the Treasury finds is generally recognized as particularly suitable for lawful purposes, and (2) an antique or a rifle which the owner intends to use for sporting purposes. The bill authorizes certain temporary interstate transfers of firearms for lawful purposes.