Q: Can a person that was convicted of felony charges (and has served)purchase/use a muzzleloader in the State of Maryland.?

A: (1) the prohibition only applies to "disqualifying felonies" -- so that needs to be addressed first, but assuming you have a disqualifying felony, then

(2) Maryland's prohibition bars a convicted felon from posessing either a "regulated firearm" (handgun or one among a list of named or similarly manufactured firearms set forth in the statute), or a rifle or short-barrelled rifle (deined as a firearm that fires or can be adapted to fire a cartidge projectile). Both defintions exclude "antique firearm" from the definition; so,

(3) does your muzzleloaded firearm fit within the definition of "antique firearm"? Maryland and federal law both apply, and the definitions are not exactly the same.

Under federal law, there are three different classes of guns that fall within the definition of "antique" under 18 U.S.C. Sec. 922:

(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.

So, a black powder muzzle loader, with certain exceptions, is not a "firearm" as that term is used for purposes of the federal Gun Control Act and its various amendments, and is not subject to all of the federal restrictions that apply to interstate transfers of a "firearm," purchase of a "firearm," etc.

Maryland law contains a narrower definition. The Maryland definition of "antique," found at Criminal Law Article §4-201, reads:

(b) “Antique firearm” means:

(1) a firearm, including a firearm with a matchlock, flintlock, percussion cap, or similar ignition system, manufactured before 1899; or

(2) a replica of a firearm described in item (1) of this subsection that: (i) is not designed or redesigned to use rimfire or conventional centerfire fixed ammunition; or (ii) uses rimfire or conventional centerfire fixed ammunition that is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade.

So the Maryland definition contains no reference to muzzleloading capacity, nor any reference to any kind of gun powder. The Maryland definition would apply to a "replica" of a pre-1899 firearm that does not use fixed ammunition, which would cover many black powder guns, but perhaps not all.

To be safe, you should contact the State Police regarding the issue and their interpretation, if any.