The same Penal Code section has the definition of "Firearm" as far as Texas is concerned, and it has nothing to do with whether or not it has a serial number or was sold:
I think if you made such a thing in Texas, your argument would be that before you put the pipes together in a certain configuration, they were just raw materials, so the "device" that you built was originally designed and made as a firearm, and therefore couldn't be a zip gun by definition. And I think that is the right answer. But I'm not a lawyer, and if I were planning to do this, I would definitly get the advice of one.
"Firearm" means any device designed, made, or
adapted to expel a projectile through a barrel by using the energy
generated by an explosion or burning substance or any device
readily convertible to that use. Firearm does not include a firearm
that may have, as an integral part, a folding knife blade or other
characteristics of weapons made illegal by this chapter and that
is:
(A)an antique or curio firearm manufactured
before 1899; or
(B)a replica of an antique or curio firearm
manufactured before 1899, but only if the replica does not use rim
fire or center fire ammunition.
I think if you made such a thing in Texas, your argument would be that before you put the pipes together in a certain configuration, they were just raw materials, so the "device" that you built was originally designed and made as a firearm, and therefore couldn't be a zip gun by definition. And I think that is the right answer. But I'm not a lawyer, and if I were planning to do this, I would definitly get the advice of one.