It's kind of hard to conscientiously let flat out misinformation stand on a respected forum.
I'm sure there are some mistaken folks who think that's what they are doing repeating "old wives tales" as well, however none of the claims here of the opinion that a loading gate creates a firearm have pointed to any federal law stating such.
See Federal Law cite below -
If the revolver in question meets either A or B or C below it is an antique firearm
Most of us are referring to either B or C below depending on whether you believe a C&B revolver is a muzzleloding pistol or not.
A C&B revolver has NOT been designed or redesigned for using fixed ammunition if it can't use fixed ammunition. Therefore a replica C&B revolver with a percussion cylinder is an Antique Firearm.
18 USC Ch. 44: FIREARMS
§921. Definitions
(16) The term "antique firearm" means—
(A) any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; or
(B) any replica of any firearm described in subparagraph (A) if such replica—
(i) is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or
(ii) uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade; or
(C) any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol, which is designed to use black powder, or a black powder substitute, and which cannot use fixed ammunition. For purposes of this subparagraph, the term "antique firearm" shall not include any weapon which incorporates 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.