It looks like TX law for public lands (at least as it applies to this particular public land) is similar to other states. They consider public land hogs to be "small game" and as such you must use "small game" weapons and loads.
OK has the same laws. In the WMAs and on some federal lands we are limited to a shotgun using BB or smaller shot or a rimfire rifle in small game season. i've killed dozens of wild hogs using a .22 magnum.