[mrerick]

They do specifically define an antique. It has to be made prior to 1899 and doesn't use modern ammunition:

431.3 Antique Firearm

An antique firearm (including one with a matchlock, flintlock, percussion cap, or similar type of ignition system) is any firearm manufactured in or before 1898, or a replica of such a firearm, that meets either of the following conditions:

1. It is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition.
2. It uses rimfire or conventional centerfire fixed ammunition that is no longer manufactured in the United States and that is not readily available through commercial trade channels.

And 12.1.1.h:

h.
Antique firearm
means any muzzle loading rifle/shotgun/pistol, which is
designed to use black powder or a black powder substitute, and which cannot
use fixed ammunition (except those that incorporate 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); or any
firearm (including those with a matchlock,
flintlock, percussion cap, or similar
type of ignition system) manufactured on or before 1898, or any replica thereof,
if such replica:
1. Is not designed or redesigned for using rimfire or conventional centerfire
fixed ammunition.
2. Uses rimfire or conventional centerfire fixed ammunition that is no longer
manufactured in the United States and that is not readily available in the
ordinary channels of commercial trade.