Muzzleloaders and black powder guns usually refer to antique or "curio" firearms. These types of weapons are excluded from the legal definition of "firearm" under Texas and federal law.
Texas defines "firearm" in Section 46.01 of the Texas Penal Code. This definition has an exception for antique firearms in (3):
Firearm does not include a firearm that may have, as an integral part, a folding knife blade or other characteristics of weapons made illegal by this chapter and that is: (A) an antique or curio firearm manufactured before 1899; or (B) a replica of an antique or curio firearm manufactured before 1899, but only if the replica does not use rim fire or center fire ammunition.
Similarly, federal law defines "antique firearm" in Section 921 (a)(16), Title 18 of the U.S. Code. Much like the Texas law, "antique firearms" are not included in the legal definition of "firearm." See Section 921(a)(3), Title 18 of the U.S. Code.
Texas and federal laws restrict people convicted of certain offenses from possessing firearms. These laws are in Section 46.04 of the Texas Penal Code and Section 922, Title 18 of the U.S. Code, respectively.
If antique weapons aren't considered firearms, can people with felony convictions possess them? The Federal Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) addresses this in an FAQ:
The Gun Control Act of 1968 (GCA) prohibits felons and certain other persons from possessing or receiving firearms and ammunition (“prohibited persons”). […] However, Federal law does not prohibit these persons from possessing or receiving an antique firearm.
However, certain muzzle loading models of antique weapons are classified by the ATF as firearms. This means they cannot lawfully be possessed by "prohibited persons." Please read the full text of the FAQ for more details and a list of weapons classified as firearms.
Additionally, the ATF has illustrated examples of antique firearms on their website.
The law can be complex, so you may wish to talk to an attorney about whether you can legally possess a firearm. For more information on finding an attorney, please see the library's Legal Help page.