[SJackson]

I wondered about that, though my thought was the skill necessary to build it. Also the “registered” comment, CA doesn’t register firearms, do they?

[Axenolith]

80% receivers often come with templates that make them pretty easy to finish.

[grobdriver]

CA doesn’t register firearms, do they?

All FFL sales result in a "registered" firearm that can be tracked to you, the buyer. The FFL keeps the records for at least 5 years (most longer), Form 4473.
At present that information is not data mined by the fed... that we know of.

And they've tried to "register" ammunition as well, including a background check to purchase.

Crazy!