Hm. Well, looking at the Equipment Availability table (CSC2023, page 4), I'd personally rule that all starship-grade weaponry is considered "specialized equipment", which means a -1 DM on the check right there. In addition, any weapon system other than a laser or a sandcaster is going to be considered military or paramilitary, for another -2 DM. (Yes, missile launchers too. Standard missiles might or might not be considered paramilitary, but those launchers can also be used to fire some much nastier and more illegal ordinance.) And the other possibly applicable DMs from the table can also contribute - world population, starport, tech level, trade codes, and the Traveller's willingness to pay more.
Now, the check for availability is normally an Average (8+) Broker or Streetwise check, but I would rule that it was only the case for turret weaponry. For barbettes, I'd call for the check to be Difficult (10+), small bays would mean Very Difficult (12+), and so on.
Now, as far as black (or at least grey) market purchases of starship weapons go, I'd classify lasers and sandcasters as Category 2 (Civilian Use), most other weapons as Category 3 (Paramilitary Use), and anything with the Radiation trait as at least Category 4 (Military Use), possibly all the way up to Prohibited (nuclear missiles and torpedoes, definitely!).
Now, the check for availability is normally an Average (8+) Broker or Streetwise check, but I would rule that it was only the case for turret weaponry. For barbettes, I'd call for the check to be Difficult (10+), small bays would mean Very Difficult (12+), and so on.
Now, as far as black (or at least grey) market purchases of starship weapons go, I'd classify lasers and sandcasters as Category 2 (Civilian Use), most other weapons as Category 3 (Paramilitary Use), and anything with the Radiation trait as at least Category 4 (Military Use), possibly all the way up to Prohibited (nuclear missiles and torpedoes, definitely!).