[seany]

The serial number part of this comment is only true if you want to sell a firearm you've made. If you built it for yourself undersized is perfectly fine (at the national level).

You also only have to apply for anything unless you become a manufacturer.

[Rebelgecko]

In California you have to buy serial numbers from the government, even if the manufacturing is just for personal use. There's also laws that make it illegal to make guns out of certain materials, and any pistol/handgun is basically required to be single shot/bolt action.

[seany]

Yeah, I was only talking nationally. That said I think CA is the only state like that, so long as you are otherwise allowed to own firearms.