Texas probably has a barrel and/or overall gun length limit. In Nebraska a shotgun has to have a barrel of at least 16inches, But the Shockwave has been classified by the ATF as a "firearm" not a shotgun. So its not technically illegal in Nebraska. I guess Texas must define shotgun in such a way that Shockwave is classified by the state as one.

I believe due to texas laws, they can ship any gun to texas, depending on if its a class 3 or not. For Class 3 firearms you need a class 3 ffl license.