If it's really an assault rifle (Title II - full auto firearm), then you will have to find out if that's legal to own in Texas. If so, pay the NFA transfer tax (form 4?) and have it shipped to a Title II dealer in TX. - I think I have that right, if I don't, someone will be along shortly to correct me.
If it's just a scary black rifle, then you will need to have the rifle shipped to a TX FFL and fill out form 4474 for the transfer.