Regarding smuggling ammo into California: how would anyone know where you bought your ammo unless they saw you, you told them, or they found a receipt?
1. It’s legal to possess in California.
2. It’s legal to leave the state with your ammo.
3. Why would it not be legal to return to the state with your previously possessed ammo?
I can’t see a law like that being terribly enforceable or effective at all. I think it’s simply to harass gun owners, and “do something” about guns.