Sorry you're having such difficulties. Those people need to learn their jobs better. From the BATFE website....
"The term “antique firearm” means any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898.
The definition includes any replica of an antique firearm if it is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or uses rimfire or conventional centerfire ammunition which is no longer manufactured in the United States, and which is not readily available in ordinary channels of commercial trade. Further, any muzzle loading rifle, shotgun, or pistol which is designed to use black powder or black powder substitute, and which cannot use fixed ammunition, is an “antique firearm” unless it (1) incorporates a firearm frame or receiver; (2) is a firearm which is converted into a muzzle loading weapon; or (3) is a muzzleloading weapon which can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof. See 18 U.S.C. § 921(a)(3), (a)(16).
Thus, a muzzle loading weapon that meets the definition of an “antique firearm” is not a firearm