Dis-assembled seems not to matter in most places.
Using the Federal definition of firearms as the example, 18 USC 921
(a) As used in this chapter—
(3) The term "firearm" means
(A) any weapon (including a starter gun) which will or is designed to
or may readily be converted to expel a projectile by the action of an
(B) the frame or receiver of any such weapon;
(C) any firearm muffler or firearm silencer; or
(D) any destructive device.
Such term does not include an antique firearm.
no mention is made of 'operability' or 'completeness'. It would appear that, generally, firearms transportation laws apply to a firearm in whatever state of mechanical assembly/disassembly it may be. Your state law may differ.
In the instant case of California, handguns are different from long guns. Both must be transported unloaded, but handguns, and 'registered assault weapons', must be transported in a locked case. No rules apply to transporting ammunition in relation to a gun, beyond 'unloaded'.