Expanse Ship Weapons

I had originally done a page about ship weapons in The Expanse, but I recently reread books 1-6 and realized how biased it was by the TV show. As cool as the gatling gun PDCs of the TV show are, they're unrealistic in many ways. And I picked up several examples in the book that are ignored/changed in the TV show to make it visually cooler. For example, Holden mentions in Leviathan Wakes that PDC rounds go several thousand meters per second, which is several times the velocity of a modern high velocity autocannon round. Granted, it's the future and they could have higher velocity cannons, but railguns make more sense. When transferring ammunition from the MCRN ships in Caliban's war, Holden muses that a 20,000 round crate of PDC rounds weighs over 500 kilos or about 25 grams/one ounce per projectile. And actually, it would be less, since the crate itself would be pretty heavy to hold half a ton of projectiles. In Cibolla Burn, Havelock is on the hull of the Rocinante and the PDC mount is described as "…a single thick barrel… The hole on the end was a black dot small enough that Havelock could have blocked it with the tip of his ungloved pinky finger." As pretty as watching the gatling PDCs in the show is, for my game, I'm trying to be more by the book. Also the TV show wiki describes the PDC guns as being 40mm, which is very large for a gatling style autocannon.
So it's more reasonable that the PDCs fire very small, super high velocity railgun rounds. After all, how much of a projectile do you need to disable a missile enough to keep it from hitting? As for PDCs being used in an anti-ship mode, that's fine if you're shooting up unarmored freighters and possibly even lightly armored escort vessels at very close range. Even at long ranges though, you have to remember the ships are probably moving much faster than the rounds and the impact of the round is going to be colossal.

Expanse Ship Weapons
Railgun
Spinal/Keel mount for light warships. x4 Newtech modifier. D6W Ship scale damage. 1 ton, 80,000 credits. Rounds weigh 10kg, cost 80 credits per, ignore 2W of armor. Unit consists of weapon with a 100 round magazine and battery pack.
Total unit- 3 tons, 80,000 credits. 8,000 credits for full magazine. (I need to work on it. This doesn't come out particularly massive built with the Serenity rules)

Light Plasma Torpedo
Usual torpedo for frigates and destroyer class ships, sometimes used on smaller light cruisers. D10W Ship scale damage Plasma warhead, ignores 4W worth of armor. 1 ton, 2,500 credits. Launcher weighs 1 ton, 100 credits.

Light Standard Torpedo
High explosive warhead. D4W Ship scale damage. 1 ton, 2,050 credits.

Medium Plasma Torpedo
Usual torpedo for most light cruisers and heavy cruisers. D12W Ship damage scale Plasma warhead, ignores 5W worth of armor. 2 tons, 5,000 credits. Launcher weighs 2 tons, 200 credits.

Basic PDC Cannon
x4, 400cr for weapon, d4 vehicle damage, 50 kilos.

New Weapons
I feel something needs to be added to the list of weapons. So, for my Expanse canon, I will add some more options. Epstein drive powered torpedoes are the preferred weapons of real warships, but pirates and such need smaller, cheaper and more easily concealed weapons. Conventional cannons firing shells may seem odd in the Expanse setting, but when your target is a slow freighter, even cannon rounds can hit your target. Also, simple chemical rocket propelled missiles are another, perhaps more reasonable, alternative. With the level of fabrication technology available, making simple unguided rockets or even basic guided missiles is possible. Chem fueled projectiles are cheap, difficult to detect and hard for PDCs to intercept. At close range, a salvo of rockets can rip apart a freighter or even a light warship. A multiple rocket launcher could be concealed in an airlock or cargo bay and exposed for firing at the last moment when the target ship is too close to take defensive measures.

Weapon lasers possibly exist as infantry heavy weapons or vehicle mounted versions on tanks and such, but that's total supposition. Considering the extreme ranges of space combat, lasers may simply not be effective as ship weapons. I would expect them to be good point defense weapons, but the authors went with projectile weapons instead.
In Leviathan Wakes, there are automatic defense lasers on Thoth station during the OPA boarding action. The troops had anti-laser gas grenades, suggesting laser weapons are common enough that even the poorly equipped Belter assault troops had the gas grenades. They mention that the docking tube on the Rocinante is armored against lasers and such. Perhaps lasers are so power hungry they are only used in fixed installations where they can run off of massive power plants. Then there could be lasers on capital ships. I may have to sit down with the Striker miniature rules from Traveller and compare railguns and lasers for power efficiency. And maybe tweak the Striker stats to use as a guideline.