The ATF link to the proposal is here:
https://www.atf.gov/firearms/docs/r...frame-or-receiver-and-identification/download
Looking at 27 CFR 478.11 (at p. 322) contains the following as part of the definition of "firearm":
"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." [emphasis added]
So no, the proposed rule does not convert your muzzleloader into a "ghost gun", the antique firearm class being excluded (those being the guns that do not fire fixed ammunition, or have a manufacture in or before 1898, etc.). (The question about conversion cylinders is a different case, where the conversion cylinder moves the revolver from an "antique" to a conventional "firearm".)
What concerns me is how the proposed rule hones in on the concept of anything that can be, "designed to or may readily be completed, assembled,
restored, or otherwise converted to expel a projectile by the action of an explosive." In other words, we are now hanging our hats entirely on that antique firearm classification. And the concern also is this is being done as an administrative rule, not as true legislation.
In other words, what is to keep the ATF from trying to come back and narrow or emasculate the "antique firearm" category? One would normally say that the NFA '34 and GCA '68 would set the definitions, but if the name of the game is to use an administrative agency to usurp a legislative power, then I would keep a close eye on the agency docket because separation of power no longer has any meaning.
The notion that the framers of the Constitution ever intended such an abomination as agencies usurping the legislature is absurd. James Madison would be astounded by the power these administrative agencies have accrued. Many fundamental rights are in jeopardy when the law is whatever an administrative agency's marching orders are. I keep these sorts of things in mind whenever I head to the polls.