If the firearm was truly made before 1898 then it is an antique per the NFA of 34 and GCA of 68 and does not meet the federal standards as a firearm, even if it uses fixed ammunition. Replicas or anything made after 1898 that uses fixed ammunition that is readily available IS a firearm and requires a background check when bright from a dealer.
https://www.atf.gov/firearms/firear...arms-ammunition-gun-control-act-definitions-0
https://www.atf.gov/firearms/firear...rms-national-firearms-act-definitions-antique
Here is an example.
An original Colt chambered in 45LC made before 1898 can be sold all day long without doing a background check while a modern replica of the same gun does require a BG check (if bought from a dealer).
As always check your state laws! Some states have more strict laws and require background checks to be done on antiques, muzzle loaders and even air guns.