Generally from what I've seen:
- missiles are anywhere in range
- anything in a turret (cannons, light lasers, railguns, etc) is anywhere they have LOS to, but heavier turrets turn reeaaallly slow so anything not close to immediately-in-front might move away too fast
- heavy lasers (the only "mounted"/"spinal" weapon so far) are as you said

Growing weary of the various downsides to nukes (possibly I overused them, twelve isnt too many for a single ship?) I switched to a railgun design for my flagship.

I can testify that heavy railguns can NOT target anything in LOS. Their "Z" axis tilt, that being their "up and down" tilt -- or more specifically their "DOWN" tilt is very minimal.

My ship is a little bit "tall" and I can't target ANYTHING unless I tilt the ship steeply while pointed at the target I want to shoot. I've got a line of heavy railguns on the top and bottom, so I spend all my time sea-sawing up and down.

I could solve the problems in the design by making it less tall, but the basic issue is that the guns can't tilt "down" very far, and ther is no indication of their fireing arc anywhere.

A visualization in the construction system would be ideal, but I'd settle for being able to read it in a file somewhere and then eyeballing if my ship had 15 degrees of clearance, or whatever.

Luke you have indeed interesting choices of weapons ;D those are the exact two weapons I never used (railguns) and used only once for a short time (nukes) until I came to the conclusion that they are vastly inferior to every other weapon I could possibly use
The railguns already look like they are likely to be unmovable, so targeting becomes more difficult the bigger (and slower) the host ship gets and taking the size of the heavy railguns I can't imagine an effective design for them. If they would at least do a megaton of damage to compensate for the vastly higher dificulty in aiming, I might have an idea or two, but in it's current state... I don't even think a straight forward weapon with little or no firing arc is a good match with the rather passive aiming system in the game.
And nukes fly too slow, reload too slow and are shot down too fast to be even called decent in my experience... I thought they were cool when I first found them, but well :|

A visualization in the ship editor would be nice indeed. I don't like it much to praise another game here, but Robocraft does that with it's turrets too, so if there is need for some inspiration on how it could look like I could provide a few screenshots.

I'm with Xelan - I never use railguns, they seem straight up worse than Dual Cannons and Howitzers. Both of those have complete firing arcs to anything they can see in LOS.

But a firing arc overlay in the ship editor would be nice, among other changes mentioned elsewhere in these discussions to make weapon choice/placement a bit easier.

Honestly, I played the first 90% of the game with nothing but Cruise and Normal Missiles.

Nukes are great for wiping out crew, I picked up a ton of carriers from the various factions that way. Less usefull when going up against PD heavy pirates, whom use my own designs.

The reason I moved away from the missile platform, was needing a reliable and fast way to kill pirates. Standard missiles would have done the job, but lack range (with instant jump, they might be the best thing though). Crusie missles are too easy to out-run, and Nukes are too easy to shoot down.

I went with the Heavy Rail because they have instant flight time, good range, and high damage. and their upgrade has some crew-kill feature.

cannon type weapons have projectile flight time, which means that a small number of their shells hit fast ships. I figgured a howitzer would be at least as slow to aim as the heavy rail, and the long flight time combined with high distance would probably negate its advantages. Maybe not?

I'll have to try some other things out. Lesson for now:

ANY turret on a tall ship that is high above the normal plane of the system, will have a hard time aiming down. Build large ships such that turrets are close to the plane of the system, so they do not have to tilt.

Good thing I tend to build ships relatively flat :D Regarding howitzers, they aren't that slow in aiming for their target and they are pretty accurate too. Way more accurate than the dual cannon and even at max range at least 80-90% of the projectiles hit their target, if that target is at least near the size of a carrier. And everything smaller than that isn't worth the attention of a battery of howitzers anyway. Thats where missiles and/or heavy lasers are shining.
The problem with the normal missiles in late game is indeed the range, which makes them pretty hard to use with an ever slower going ship. That's why I only have a battery of about 10 silos, which I use to pulverize everything below carrier size. I just wish the autofire option for turrets would actually do something. But I think I'm gonna start a new thread for that one and my sugestion for improvement.
Rails are in a somewhat strange place. They are assentially sniper weapons with okay-ish damage, but while I like sniper weapons I don't think they fit the combat and flight system. The aiming is too indirect, the flight control too restricted to use them properly.

Let me provide some examples.

This ship cant shoot anything, unless I fly it way out of plane. Then half its turrets can target:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=815026979

I could solve this problem by putting all the Heavy Laser turets on one side, and just cruising out of plane to snipe things. That would have some other complications though, and might make combat more of a PITA than it needs to be.

This ship, it can target things much better since it is both less "tall" and the railguns have some SLIGHT tilt to them. Still, I really cant shoot anything without pitching up or down. Though an improvement over the above, it is NOT an effective design:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=815026842

Chompy here is a simple improvement on Ridgey, and since the turrets are close to the main plane of combat it has a vastly easier time bearing all weapons on a target. Still, the turrets do not pitch much and if a target is off the plane, the whole ship needs to turn and pitch to bring them to bear:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=815026933

hehe. Jelly here is fairly effective. Just need to immediately fly "UP".

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=815048553

That escalated quickly

I couldn't leave it just looking like a big grey block forever:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=818188852

My eyes!

Just make the front of the ship wedge-shaped and put the guns on the slopes. Then point the ship at whatever you need dead.

See: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=812571333&searchtext=
You can't see it in the screenshots, but there's lots of gun and railgun turrets on the frontal white stripe. Easy to aim with that one.