Here, north of the razor wire, a firearm must have been made prior to 1898 to possibly qualify as an antique. All pre '98 powder and ball guns are antique.
It isn't that simple, though. Post '97 touchhole longarms are antique; cap guns aren't. No post '97 handgun is antique, no matter if match, flint or cap fired. There are other factors, many pre-1898 guns are not considered to be antique. For example, no centerfire repeating rifle is antique. With pre '98 single shot centerfire rifles, antique if over 8.3mm bore, modern if under. No .22 rimfire firearm is antique. For breechloading shotguns, antique status is determined by gauge. 10-12-16-20-28-.410ga guns aren't antique. Pre '98 centerfire handguns may or may not be antique depending on the cartridge accepted.
As Old Ford2 has pointed out, a modern flintlock pistol without a touchhole isn't a gun. It is a replica. Replicas of modern guns are prohibited; replicas of antiques are nothing.
Firearms can be registered online. Given that the project under discussion is a flintlock, inspection by a verifier probably wouldn't be necessary. Photos might suffice.