In California, muzzleloaders and cap and ball revolvers etc are not firearms UNLESS you are talking about consealed carry... Then they fall into the firearm category. I think the 18" barrel on a shotgun law might also effect muzzleloading smoothbores, but there seems to be some wiggle room because smoothbore perc and flintlock pistols are legal in CA.
Page 4 (electronic page 9):
http://ag.ca.gov/firearms/forms/pdf/Cfl2007.pdfExceptions
The term firearm does not apply to a federally defined “antique” firearm for the purpose of dealer licensing requirements, sales or loans between private parties, or requirements to obtain a Handgun Safety Certificate pursuant to Penal Code sections 12070, 12071, subdivisions (b) (c) or (d) of 12072, or 12073. The term firearm does not apply to federally defined “curio” or “relic” long guns over 50 years old for the purpose of transfers between private parties. (Penal Code §§ 12001(e), 12078(t)(2).)
18 USC 921:
http://www.law.cornell.edu/uscode/uscode18/usc_sec_18_00000921----000-.html(16) 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.
