I don't know how the zip gun works, but I do know how parentheses work. You use parentheses in pairs to enclose words that clarify or are used as an aside.
Never bring “parentheses” to a gun fight.
Everyone else may use them that way, but in point of fact, he did not.
One thing it certainly would be is patently dangerous.
No safety features on it that I can discern, and it is liable to blow up in your face on the first round.
Unless that section of pipe is “rifled”, then that firearm would be considered to be a “short barreled shotgun” (A shotgun has to have at least an 18 inch barrel and an overall length of around 27” (not real sure about that) or it’s the same as a sawed off shotgun and the BATFE can pitch you in Club Fed for merely possessing it.
Even the few surviving specimens of the “Liberator” being smooth bored have to be registered with the BATFE the same as a machine gun.
Fooling with this sort of gizmo can be fun, but is it worth about $200,000 in fines and legal fees, 10 to 15 years of your life and a lifetime felony conviction that would prohibit possession of any firearm?
Not for me it ain’t!
Of course any career criminal who wants to make one to use for a crime probably isn’t too worried about the BATFE.
Besides; real guns are readily available on the black market and not all that hard for a competent thief to steal.