This is a horrific disgrace to English, but if we break it down and subtract irrelevant parts, what it does is tell us what an "antique" is (*) and also creates an exception for firearms that ordinarily would not be antiques.(1) "Antique firearm" means a firearm or replica of a firearm not designed or redesigned for using rim fire or conventional center fire ignition with fixed ammunition and manufactured in or before 1898, including any matchlock, flintlock, percussion cap, or similar type of ignition system and also any firearm using fixed ammunition manufactured in or before 1898, for which ammunition is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade.
To be an antique the firearm must meet two elements:
- not designed or retrofitted to use rim/center fire cartidges AND
- manufactured before (EDIT: "in or before") 1898
(*) For purposes of the statute -- this definition differs from the colloquial meaning of antique.