There are origins of definitions that are creating the confusion. For this discussion, per the ATF, there are Firearms and Antique Firearms.
- Antique firearm.(a) Any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; and (b) any replica of any firearm described in paragraph (a) of this definition if such replica (1) is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or (2) 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.
- Firearm.Any weapon, including a starter gun, which will or is designed to or may readily be converted to expel a projectile by the action of an explosive; the frame or receiver of any such weapon; any firearm muffler or firearm silencer; or any destructive device; but the term shall not include an antique firearm. In the case of a licensed collector, the term shall mean only curios and relics.
This is a little confusing, but the fact is the Encore is a firearm. Whether a rifle barrel, shotgun barrel, or muzzleloader barrel is affixed, it is a firearm as defined by the ATF. A firearm requires the purchaser to complete ATF Form 4473 when purchased new. A firearm is a regulated item having manufacturing, shipping, distribution, storage, sales, and removal from use restrictions.
States may allow the use of a Firearm for hunting as a muzzleloader when the proper barrel is affixed, but affixing a muzzleloader barrel to the registered/serialized receiver
does not convert the Firearm to an Antique Firearm. Once a Firearm, always a Firearm, until it is destroyed.
The next issue is that each state defines what is considered a legal muzzleloader for their game seasons. A good reference can be found here.
Muzzleloader Regulations by State Caveat is that these may or may not be up to date. Many states, in fact the majority of states, do not require the item to be charged by adding the powder at the muzzle end for muzzleloader hunting seasons.
The ATF define what a Firearm/Antique Firearm is, the states define what type of item is allowed to be used in their muzzleloader season, and the individual defines if they want to restrict the state's definition based on their own beliefs. America - no other place like it.
Hope this helps.