Legal Question in Criminal Law in California

Yes. If they weren't, you wouldn't be able to buy them in the many stores where they are available, or have them shipped to CA from internet purchase. Their use is regulated just like any other weapon.

A comment on Mr. Nelson's answer:

I don't know whether these pistols are legal, but I presume Mr. Nelson is right. Even so, the fact that something is easy to purchase in California does not mean it is legal. Brass knuckles, drug paraphernalia, and various other illegal items are available online, by mail order and in many stores, but that does not mean you can buy them legally. Their availability might *suggest* that they are legal, since law-abiding businesses would not offer them. But it does not prove the point, since some businesses are not law-abiding. Additionally, businesses that try to obey the law sometimes violate it unintentionally.

I point this out because many people presume anything they are able to buy at a retailer must be legal. That belief sometimes gets people into trouble.

You can own a CO2 power air pistol in California. What you do with it is another story.