RAW you get one spinal mount which is aimed primarily by aligning the craft itself. However, I can imagine a house rule variant that placed spinal mounts in huge turrets. Coming up with enough power for them would be the hard part.
If it's an issue for a power plant that's rated for a 20,000 ton ship, how hard can it be for a plant that can power a 500,000 ton ship?
Wet Navy Battleships (in RL) ranged from 10,000 tons to 65,000 tons according to wikipedia, and the average was about 32,000. Iowa, a giant, was 57,500 tons, but the big16-inch guns were 121 tons each, so about 2.5 percent of the ship's mass. Proportionally that's the size of a 5-ton barbette on a 200-ton ship, though they mounted nine in three turrets. That said, a TL13 Meson D is 14,000 tons (in Mongoose 2008, sorry, it's the only book I own). That Meson D mount is the same relative size as the battleship's gun when compared to a 500,000-ton Traveller dreadnaught: about 2.8%.
Spinals are not spinal, IMO, unless they take up more than a fifth of a ship's tonnage. That's the anime-esque visual it seems like the RAW were going for. The A-10 aircraft with the Avenger autocannon is probably closest in RL to a spinal-style mount. Wiki says the entire gun system represents about 16% of the aircraft's mass, so maybe 1/6 of a ship's mass makes a weapon spinal?
It seems wildly silly to me that a gun that's 2.8% of your mass, similar to a barbette on a smaller ship, is considered spinal, and supports the suggestion of considreing them as huge turrets.
The problem is spinals are a fixed size, and the game has made ships huge beyond the scope of sense. In RL, a battleship was about 10-20x the mass of a destroyer from that era. In Traveller, the wiki at TravellerRPG.com marks them as 1000-2499 tons, but further down calls that a 'Light Destroyer' and adds heavier destroyer classes. Some destroyer SDBs can mount spinals. A TL15 2000-ton destroyer SDB can mount a Spinal Meson A. And wouldn't that be a big surprise for an invading fleet! A battlerider carrier loaded with these could lay waste to enemy fleets. But Traveller dreadnaughts are 200x the tonnage of a destroyer, and we should expect a proportional increase in weapons. Alas, there -are- no bigger guns in the Traveller universe than the spinals, which are dwarfed by these huge ships. Again, this supports the idea of using the current catalog of tiny spinals as capital turrets.
I would also be totally alongside the idea of a 500,000-ton battleship with a 100,000-ton spinal mount, something like that deserves the name Spinal, even if it weighs more than 7 Meson D's. I imagine it hits like a death star's main weapon, which as I think about it, is just a huge spherical form factor dreadnaught with indeed, a single giant weapon. Alas, the death star, if it's the size of a small moon of 10km diameter, is about 37 billion d-tons. A 16% spinal on that would be over 6 billion d-tons, which is over 445,000 Spinal Meson D's.
That, I think, would absolutely destroy Alderaan in one shot.
But that's a ton of houseruling craziness that has diverged from the main point, that the spinal mounts in RAW are not at all what the descriptive text claims when mounted in actual dreadnaughts and some revising ought to be done.
). That's why I've been advocating for a crew quality factor in HG since ages...
