Short-barrel firearms are prohibited under Texas Penal Code which defines them as " a rifle with a barrel length of less than 16 inches or a shotgun with a barrel length of less than 18 inches, or any weapon made from a shotgun or rifle if, as altered, it has an overall length of less than 26 inches."
Texas doesn't have a definition of shotgun like the federal law does which is how the Mossberg skirts the federal shotgun definition. You'd think Texas would default to the federal definition making the Mossberg legal, but who knows.