Actually LEO's are required to actually know the laws and how they are applied. They don't have to answer legal questions, and can't be held liable if they are wrong with their answer, but the excuse "I thought that was the law" does not shield us LEO's from criminal and civil prosecution if a judge decides we had a "duty to know", nor does it indemnify the civilian who got bad information.
There are problems these days with the OP's question no matter what state..., here's why...
In the State of Maryland, we used to have convicted felons using black powder guns to hunt. In one case a man was using a repro 1858 Remington as a home defense handgun, after being convicted of a crime where he could have been imprisoned for more than two years..., this is important, folks, because some states like Maryland have misdemeanors which have max penalties of more than two year in jail. The federal laws do apply to misdemeanors with possible penalties that exceed 2 years.
According to the feds the word "felony" is not needed, just the fact that the conviction could have resulted in a sentence of more than two years . What the person received as the actual sentence is moot....and if you have "common law" as we used to have, with a vague penalty of "whatever the judge deems is reasonable"...that falls into that category too. So for a while in the 1990's men were pleading guilty to common law assault on their spouse or girlfriend, which means they "put them in fear" of being hit (while not actually striking the women) and in a day or two after the conviction the State Troopers were showing up to collect the man's guns. :shocked2:
So the guys were fine, using caplocks or flinters or even cap-n-ball revolvers for a while....,then came Cowboy Action Shooting....
The Federal Law reads thus:
18 U.S. Code § 921....
(a)(16)The term “antique firearm” means”” ...
(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. (so far so good...)
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.
So this paragraph does NOT use the term cylinder...it uses "breechblock" so in my mind they are referring to rifles. So I would argue that any of the repro antique, cap-n-ball revolvers used in CAS, which MAY be readily converted to use cartridges with a drop-in cylinder (black powder cartridges count folks) would not fall under this due to the lack of the word "cylinder", BUT ..., would an Assistant United States Attorney agree with me? Would a Federal Judge? I have had several AUSA's tell me the charge would work against a felon using an 1858 Remington with a drop in cylinder that uses .45 Colt ammo with black powder or Pyrodex. ???
..., OK so we decide to play-it-safe and the felons simply don't use cap-n-ball revolvers...
Second, you have the following, as a felon (or a person convicted of a misdemeanor with a possible penalty of more than 24 months in jail) cannot possess "ammunition"...,
18 U.S. Code § 921 (a)(17)(A)
The term “ammunition” means ammunition or cartridge cases, primers, bullets, or propellant powder designed for use in any firearm.
Well, before CAS, and the return of fixed ammunition using black powder, all of the traditional rifles, and guns, and pistols and cap-n-ball revolvers were "antiques", and black powder and pyrodex was intended for use in "antique firearms" (which means repros too)..., NOW, however, you have Black Powder being used to manufacture cartridges which may be readily used in modern firearms such as a Marlin or Winchester lever action rifle, THUS Black Powder is now "propellant powder designed for use in any firearm" and even if using a traditional rifle or gun... having BP might be a violation, and anti-gun prosecutors and judges may decide that BP or any substitute is now no longer legal for use by the convicted felon. There may be a section the specifically lists black powder as an exception to this..., I can't find it right now..., but I doubt it lists the black powder substitute powders along with the black powder.
LD