Isn't the fact that naval artillery is moving while firing a significant difference in application?
What proficiency do you feel covers a tank destroyer's 90mm cannon?
Based on the book, it's vehicular, even if it's mounted on a tow carriage.
Considering that a 12lb gun is operated and serviced the same way whether it's in a field mount or a naval mount, and that it has nothing in common with the other tactical weapons, it's a vehicular weapon.
That makes sense. One of my players is toting a 2lb cannon as is personal armament, so I made that tactical, just judging by its application.
Edit: I'm not actually sure on the weight, but its one those 1.5" bore swivel guns, that he carries around. I guessed that a 1.5" lead ball would be about 2lb, but I'm almost certainly off.
I have no problems calling a swivel gun a tactical weapon, however.