Generically a weapon made in or before 1898
CA Penal Code routinely defines Antique Firearm as those that are defined in USC 18, Sec 921(a)( 16). In at least one place, CA Penal Code defines Antique Firearm as "made prior to January 1, 1899". In one area, dealing with "destructive devices", it defines it according to [Section 479.11 of Title 27 of the Code of Federal Regulations.] 
[USC 18, Sec. 921(a)(16)] reads:
The term "antique firearm" means -
(A) any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; or (B) any replica of any firearm described in subparagraph (A) if such replica - (i) is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or (ii) uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade; or (C) any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol, which is designed to use black powder, or a black powder substitute, and which cannot use fixed ammunition. For purposes of this subparagraph, the term "antique firearm" shall not include any weapon which incorporates a firearm frame or receiver, any firearm which is converted into a muzzle loading weapon, or any muzzle loading weapon which can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof.