Does the US government have a monopoly on weapons developed here?

[sweeteviljesus]

Well, it depends on the legal definition of a weapon. If the death ray in question isn’t covered under existing laws, I don’t think there are any impediments to selling it domestically and maybe none for selling it internationally.
With respect to “born secret”:

Whether or not it is constitutional to declare entire categories of information pre-emptively classified is not clearly known. When it was directly challenged in a freedom of the press case in 1979 (United States v. The Progressive) where a magazine attempted to publish an account of the so-called “secret of the hydrogen bomb” (the Teller-Ulam design) which was apparently created without recourse to classified information, many analysts predicted that the Supreme Court would, if it heard the case, reject the “born secret” clause as being an unconstitutional restriction of speech. The government, however, dropped the case as moot before it was resolved. (DeVolpi 1981)

Rob