I do have to ask....
What are you trying to accomplish?
Ship to ship combat seems a little out of scale for T2K's individual fighter level (IMHO).
If you are trying to get individual PC's skills in the game, it would be easier to graft mods into Harpoon or Shipwreck than to reinvent the wheel to add large vessel combat into T2K - though almost any attack is due to a sum of crew actions rather than a particular individual's skill.
The weapon effects are generally beyond the top end of the scale (T2K also does not include rules for determining damage to vehicles, equipment, or people from aerial bombardment by 250-2000 kg bombs).
If you are trying to get land (vehicular) weapons to damage ships, you can find similar weapons in Harpoon (75mm = 3", 120mm = 4.7", 127mm = 5"; 155mm = 6").
If you want to go through the exercise, by all means. I do that sometimes just to see how a design sequence works compared to actual designs.
But I am curious as to what you want to get out of it.
Uncle ted
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