I don’t know about TX, but I recommend taking a picture of all your tax stamps and having them on your phone. That way there is never any doubt you don’t have the paperwork. Keep the originals in a safe–make lots of copies. Keep a copy with it.
Does the possessor of an NFA firearm have to show proof of registration?
Yes. The approved application received from ATF serves as evidence of registration of the NFA firearm. This document must be made available upon request of any ATF officer. It is suggested that a photocopy of the approved application be carried by the possessor when the weapon is being transported.
[26 U.S.C. 5841(e); 27 CFR 479.101]