^^^
Summary:
1: You can machine whatever you like to, under federal law, without any legal issues. If you are taking a CNC class and want this to be a graduation project, or whatever, go for it. It's not rocket science.
2: Ownership of the receiver is age restricted. A receiver that is not yet assembled as a rifle is a "neither a rifle nor a pistol" receiver, and as such, since it COULD be made into a pistol would normally only be sold to someone 21 or over if sold by an FFL. BUT that's only for a FEDERALLY LICENSED sale, and does not affect you. You will be OK at 18.
3: California State Law might very well require registration. Research this.
And good luck, once you start making things like this you'll never stop.
Willie
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