Right, I've just finished chapter 21, and while I've been enjoying the ride so far and will definitely continue...there are some rather silly decisions going on here. Primarily, the LasRail. Leaving aside that the converted weapon is almost certainly a coilgun not a railgun (GW not being well known for their research or sensible sci-fi or combat tactics for a reason)...sticking a lasgun on a
coil-gun makes decent sense if you've got the tech to make both small and light enough, and in-setting at the infantry scale one fires faster less-damaging shots while the other is naturally armor-piercing at the cost of RoF and ammo.
Scaling that up to vehicles? Now things are a little questionable, but for general purpose turrets (that would theoretically be used against a variety of targets) still makes sense, even as it doesn't for designated main cannons or chaff-clearing repeaters.
On a space-ship? Now I think you've lost track of what exactly you just told us a
Las-Rail is. It's one gun on top of another gun with your own frankensteined mat-sci and engineering program. You already had ship-scale lasers, and ship-scale coil guns? Have VERY different properties and limitations. First and foremost, the ammo is now big enough that placement of the magazine is a real concern, as is the question of a magazine period. Fabricators might be able to make your ammo for you in the barrel, but a multi-ton slug of the densest ferro-magnetics you can make still takes time that even a small magazine of pre-made slugs would rapidly outpace reloading and refiring. After-all, capacitors and charge-time aren't a concern with constant access to the whole energy net. Just the time it takes to reload a slug, and the wear/heat on the coil-barrel.
Which means that 'Rail' Cannons? Are very heavily influenced by where you put them in a ship to maximize barrel length (and thus how fast the slug can be accelerated) and convenience of ammo transportation to the cannon assembly.
Lasers on the other-hand? Don't really give a shit about anything but the emitter, focusing array, and the capacitor running it. The latter is a non-issue, and the former don't take nearly as much space.
In other words...why would you stick them on top of each other scaled up to capital-weapon sizes?
For that matter though, if you're enamored with using magnetic accelerator weapons for your (knife-fight) space battles...why on earth would you go with Star-Trek? With the exception of the Borg, every ship is a thin wavy bit of non-sense, and their weapons are only practical because they are exclusively beam-weapons with small emitters that need no turret or barrel, or (mostly) self-propelled torpedoes and missiles. Well, that and structural stability is a non-factor because of BS inertial dampening and 95% of a warships durability against ALL weapons in the setting being in their shields. The opposite of what you're gonna find for all the future capital-scale weapons like macro-cannons, plasma launchers, particle cannons, and possibly even friggin Gauss cannons (if you can ever steal those, the infantry version had a big-ass barrel, which raises some concerns about the naval version and the fact that Necrons have minimal need for internal spacing in their ships like Rex). Both in terms of the geometry demanded by the massive turrets, barrels, and magazines of naval weaponry...and in terms of a ship-structure that can deal with getting hit by giga-tons of force, or hyper-thin beams of energy that cut through shields and armor (looking at the DE and Necrons here, and while zero internal space might make dealing with blunt force from Imps and Orks reasonable, exotic screw-you beams that can cut through dozens of meters of armor go from bad to horrifying when the struts holding a massive battleship together are so relatively thin.)
Finally we come to the Ion cannons...and look. IRL, 'ion cannon' is sci-fi nonsense. But in classical sci-fi depictions? It's the non-penetrating electrical system damaging weapon. The one that doesn't bust the hull, but does fry or 'stun' or even power down another ships internal powered hardware. In some depictions it's permanent damage, in others it's mysterious power-drainage, and in most it's extra-effective on energy shields.
How did that become
there would instead be powerful ion cannons, weapons more than capable of melting through the hulls of enemy ships
?
Like, pump enough energy into a particle projector of any kind, and assuming it doesn't break, yeah it's going to start melting whatever it hits. But it's just weird, especially for a Star Wars buff where Ion Cannons are a thing in that setting, and melting hull armor is absolutely not what they do there.
Edit: The thing about the ship types is that they work in Star-Trek for a reason. There? Weapons and Shields technology is so wildly beyond armor and mat-sci that the ship design really isn't that much of an issue. If the shields fail, no amount of armor or redundant engineering is going to save you from a naval phaser carving you to pieces. And ramming and (physical) boarding are basically non-existent tactics. In 40K, comparably effective shields are either Warp-based, lost Archeotech, or Necron. And the armor is Adamantium, Necrodermis, and Wraith Bone - materials that CAN stand up to all but the most exotic attacks in the setting so long as they're thick enough.
The D'deridex sounds like it's big enough to not be so badly off for it's horizontal shape and thinner struts, especially since nearly all of it will be a solid block. But all the smaller ships? Seeing as you DON'T have OP shields yet, it just looks like a repeat of the dumb, weak, swarm tactics Rex used earlier with his bots. And while it was viable because sheer industry can replace the losses...he already learned from the Necrons that tactic only works up until it doesn't at all.
