Chris, what the DOJ may be talking about is the fact that you finished an 80% lower. Even though Federal law allows homebuilts for personal use, I don't think that CA law allows for it. I think it falls under the definition of a "zip gun".
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 zip gun,...
(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.
CA DOJ website http://caag.state.ca.us/firearms/index.html