It's not that uncommon for convicted felons to mess around with muzzleloaders since, for the most part, they're not considered "firearms". I've had several ask if they could own one to hunt with. "Whatever your parole officer says."
Most parole officers are not versed in the section of Federal law defining muzzleloaders and antique firearms. Most of them simply lump them into the "destructive device" category and don't care to learn otherwise. Having said that, such exceptions do not absolve someone of a crime such as deadly conduct as the person being robbed was still put in fear of their life. Same thing if you rob a convenience store with a steel pipe that approximates a gun barrel, your intent is the same. The person or institution being robbed is still placed in fear and doesn't have to know if it's a cap and ball revolver or not.
PS- I'm not an attorney and this should in no way be used as legal advice.
Title 18 USC.......
(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.