California Penal Code:
12020. (a) Any person in this state who does any of the following is punishable by imprisonment in a county jail not exceeding one year or in the state prison:
(1) Manufactures or causes to be manufactured, imports into the state, keeps for sale, or offers or exposes for sale, or who gives, lends, or possesses any cane gun or wallet gun, any undetectable firearm, any firearm which is not immediately recognizable as a firearm, any camouflaging firearm container, any ammunition which contains or consists of any flechette dart, any bullet containing or carrying an explosive agent, any ballistic knife, any multiburst trigger activator, any nunchaku, any short-barreled shotgun, any short-barreled rifle, any metal knuckles, any belt buckle knife, any leaded cane,[b][blue] any zip gun[/b][/blue], any shuriken, any unconventional pistol, any lipstick case knife, any cane sword, any shobi-zue, any air gauge knife, any writing pen knife, any metal military practice handgrenade or metal replica handgrenade, or any instrument or weapon of the kind commonly known as a blackjack, slungshot, billy, sandclub, sap, or sandbag...
...(10) As used in this section, a "zip gun" means any weapon or device which meets all of the following criteria:
(A) It was not imported as a firearm by an importer licensed pursuant to Chapter 44 (commencing with Section 921) of Title 18 of the United States Code and the regulations issued pursuant thereto.
(B) It was not originally designed to be a firearm by a manufacturer licensed pursuant to Chapter 44 (commencing with Section 921) of Title 18 of the United States Code and the regulations issued pursuant thereto.
(C) No tax was paid on the weapon or device nor was an exemption from paying tax on that weapon or device granted under Section 4181 and subchapters F (commencing with Section 4216) and G (commencing with Section 4221) of Chapter 32 of Title 26 of the United States Code, as amended, and the regulations issued pursuant thereto.
(D) It is made or altered to expel a projectile by the force of an explosion or other form of combustion.
William,
Would your lowers meet all four criteria to qualify as a zip gun?