Similar playbook to abortion and voter id laws in some respects.
Unless you can find a CA compliant firearm in NV/AZ, meaning max 10 rd mags and no semi auto center fire rifles with assault features, at a minimum, that’s a no go. Importing ammo to CA privately is also generally prohibited.
Being honest, I travel once a year to visit family in AZ and pick up both cf and ap rounds on my way back, never once had an issue. Even got checked once and they didn't care when I told them I had 8 boxes of ammo. I buy .380, 9mm and 12ga rounds only though. Never bought mags, but I doubt the sellers would care and my only risk is getting checked by some hard liner on my way back.
