When people think of Texas, guns tend to come to mind. Contrary to urban legend though, the state of Texas has a lot of rules regarding any sort of firearm use. In many cases, even the word "firearm" can be expanded to extremely broad definitions...which means that not only do the laws apply to pistols, rifles, and shotguns, they also apply to air guns and in some cases to BB guns as well.
According to the definitions provided by the Texas State Parks Department, air guns fall under the definition of arms and firearms. This category includes any sort of weapon by which a projectile is fired based off of an explosion, from compressed air or gas, or by some mechanical means (i.e., a compressed spring). It is against the law for a person to display or use any device that falls under the arms and firearms definition in a Texas state park unless that person is participating in a hunting activity, or they have written authorization to have and use that item.
If a person is firing a BB gun within their own home or on private property, it is not considered any sort of offense. While the force of a BB fired from one of these "weapons" may be enough to break glass or dent drywall, it isn't usually enough to cause actual injury. As long as a person doesn't attempt to cause injury to another person or damage any property that isn't theirs with the BB gun, there is no real restriction on using a BB gun on private property.
Concerning replica, or realistic, BB guns (usually called Airsoft guns), there is a federal restriction that states the guns must have an orange tip marking them out as BB guns. If a person is found in possession of a replica BB gun without this orange safety tip, it is both unsafe (as it makes them look armed, which has lead to misunderstandings with police in the past) and illegal. Texas law doesn't disagree with this federal standard, and it requires that all Airsoft guns have this orange tip when sold or owned.