Is a In-line muzzle loader considered a firearm in Texas
If you mean as a convicted felon or have a domestic violence conviction or you want to buy one when to young to buy a real one and that it is a black powder muzzle loader. You can you have one. It is still a firearm but not one that is regulated the same way as normal firearms. You cannot open carry one. You cannot carry concealed if not licensed for concealed.
As okorder /... Under USC Title 18 Chapter 44 Weapons it never states a felon can not own a firearm. It controls the commerce of firearms. So any FFL dealer can not sell one to a felon but it is not illegal for a felon to own one.
The way I read Texas law, antique or curio guns manufactured before 1899 and replicas thereof which do not use rim fire or center fire ammunition are not included in the definition of “firearm” as it is used in Texas Penal Code Title 10, Chapter 46, which governs weapons.