Interestingly as pointed out the requirement to register when you "build" or bring in from out of state are not enumerated in the law, but purely EOPS interpretation of 128B. Anyone ever been successfully prosecuted on either charge (not plead out, or similar)?
It doesn't seem like too much of a stretch to lump "build" in with "obtain". Before the build you didn't have or own a gun. After the build you did. That's not a completely insane reading of "obtain"
But that's not the case when you already own a gun and then move into the state. You already own it, so there's no "obtaining" going on.
One of the dictionary definitions of "obtain" is
To succeed in gaining possession of as the result of planning or endeavor; acquire.
The first half of that might work, maybe, but if that counts, then anytime you go to your safe to get a gun you're obtaining it.
The second half doesn't work at all, you've already acquired any gun you already own and already did a 4473 on and already have in your possession.