I've read through several old postings that clearly say it's okay by federal law, but I seem to remember something about the CA-DOJ considering homemade guns to be "zip guns".

Is that true? Or am I misinterpreting things?

caag.state.ca.us/firearms/dwcl/index.html

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.

It's so depressing to read all of that crap. The real answer is to write CalDOJ and ask for a written opinion. Of course, once you get it, post it here for us to all enjoy (yeah right).

Ok I found out some answers on this today – I talked with an annalist about this, it’s not in writing so take this info with a grain of salt

You can make an AR or AK type lower as long as it complies with penal code 12275.1(a). However, once identified as a AR or AK and placed on the list of restricted named receivers you will have 90 days to register the lower as an Assault Weapon. The Department of Justice can decide to Decline your registration of that Assault Weapon, leaving you no other choice than to relinquish, destroy or export that lower out of the state in 90 days. So much for the build it your self idea.

If you can modify the lower in major way so that it can not accept a key component such as a pistol grip, high capacity magazine, collapsible stock or AR upper then that may qualify more than a minor different. If I were you I would have that in writing before you start hacking away at a forging or casting.

1227.6(e) the term "series" includes all other models that are only variations, with minor differences, of those models listed in subdivision (a), regardless of the manufacturer.