The Airsoft instruction sheet should remind you to check your local ordinance for specific regulations. Houston, TX is city land so the older ordinance may have been updated Check the current language by a phone call City Hall. The wording was
Sec. 229.002. REGULATION OF DISCHARGE OF WEAPON. A municipality may not apply a regulation relating to the discharge of firearms or other weapons in the extraterritorial jurisdiction of the municipality or in an area annexed by the municipality after September 1, 1981, if the firearm or other weapon is:
(1) a shotgun, air rifle or pistol, BB gun, or bow and arrow discharged:
Because Houston is city property prior to September 1, 1981, any air rifle or bb gun ordinance it had would still be active.