I live in CA. Yes, you can legally bring anything that is classified as an "Antique or Curio Relic," which includes repros of black powder firearms (including percussion revolvers). Unlike modern guns, you don't have to declare it and register it. Once you live here, you can have BP guns shipped to your door without using an FFL or the waiting period.
However, the laws surrounding transportation are confusing to say the least, so it's best to treat any BP gun the same as a modern gun when transporting (keeping the ammo, powder, and caps in a separate place in the car, keeping the gun in a locked container or with a locking safety device).
My understanding is that until a percussion revolver is capped, it is not considered a firearm under CA law. BUT, I wouldn't trust any CA cops to know that.
Another thing to know is that many ranges don't allow black powder here. They exist, but many don't, probably because of perceived fire hazard.