California has many very confusing laws but the main ones to be informed of when making or building your own guns is the Assault Weapons Ban, laws regarding magazines, and others.
Calguns has a flowchart for rifles that covers most questions http://www.calguns.net/caawid/flowchart.pdf California is also one of those states that have "assault pistol" and "assault shotgun" definitions, and quite some reading will be required if you wish to comply with all the various laws.
When it comes to magazines over 10 rounds in California it is NOT illegal to possess or use them, however it is illegal to import, manufacturer, give, sell, keep for sale, etc. It is not illegal to replace parts of currently owned magazines, including every single part. As of 1/1/2014 it is illegal to import "magazine rebuild kits" however separate parts are still legal to import it would seem.
Also in California there are various standards that differ from the federal standards, for instance if you have a folding or sliding stock, the overall length of your rifle is measured with the stock closed or folded. There are many cases in CA that have set precedents that are inconsistent with federal laws and standards.
The laws regarding "zip guns" in California are very vague and are questionable, this PDF should shed some light on the problems http://calgunlaws.com/wp-content/uploads/2014/03/3D-Printing-of-Firearms-Federal-and-Claifornia-Law.pdf
As for the Federal UFA, firearms minus the stock, grips, and magazine, must have an X-ray detection signature no less than that of a calibration sample containing 3.7 ounces of stainless steel. Fun Fact: 3.7 oz. is exactly half the metal in a standard Glock 17 Gen 1, and the original bill was passed with no current firearms being affected. Also the Military and CIA are exmept from the UFA.
Do not take my advice alone, if you intend to produce your own firearms, understand your local and federal laws