This might be better served in general discussion, but I feel like there may be a rule i'm over a looking so it felt best putting it here. When putting weapons on a ship in the light weapons category:
Coilguns are 6 BP, 4d4 Long Range no drawback. 2.5 per die gives an average damage of 10
Light Plasma Cannon is 12 BP, 2d12, short range, no drawbacks. 6.5 gives an average damage of 13, along with much higher variability in damage so not as reliable.
3 damage trade off for double the BP and loss of longrange? doesn't seem worth it to me. Am I missing something that actually makes this a meaningful choice?
Overall I agree with you that the Coilgun is the better option, but as you note the plasma cannon does 30% more damage per shot - that can be worthwhile. For example a light plasma cannon has a much better chance of penetrating a carrier's Damage Threshold (DT: 10) than the coilgun.
I'd say the light plasma cannon is a decent option if you're building a mid to high tier interceptor or fighter. You'll have plenty of BP and power to spend on guns, and because of the size limitation you can't mount heavy weapons so the Plasma Cannon is the highest reliable damage output BP can buy. An interceptor's superior pilot bonus means it is likely to be able to pick its own position and engagement range so the lack of range is less of a drawback. An interceptor with two twin-linked plasma cannons (8d12) would deal a surprisingly decent amount of damage for something its size.
One weapon I do struggle to find a niche for is the light particle beam. It costs 4 BP more than a coilgun but only deals .5 more average damage, and has medium range. It uses d6s for damage to the Coilgun's D4, so it's less affected by shunting power to weapons as well.
Power to weapons is another great point, That brings the average damage for a Coilgun up to 3.5 for an average of 14 and the math isn't as forgiving with a plasma with an only 1/12 chance of rolling a 1. 1-12 damage isn't much of an improvement on 2-12. Fighters getting through damage threshold is really the only bonus since they wont typically have a gunner. I had forgotten about that.
EDIT: Forgot this part. Can you get 8d12 on an interceptor? doesn't each weapon require its own slot, you just take less actions to fire them, so 2d12 would be 12 build points per slot, 2 would be 4d12 and 24+6 for 30 BP at 4d12?
Shunting definitely makes D4 weapons a bit better, but primarily for ships that can reliably use an engineer. Most of the small ships can't fit more than a pilot and a gunner, so it's a bit less likely to come into play for them.
Edited part: I believe you can, but it's a little tricky. You put two linked plasma cannons in an arc (typically front) and another two linked cannons in turret mounts, then you use Fire At Will or Broadside to fire them all. The cost would be about 70 PB - 24 for two regular cannons, 36 for the twin links and an additional 10 for two extra turret mounts. I probably should have explained that better. :)
I agree that Coilguns are superior. The lower BP just amplifies that. Our AP group (+PFS too) has often used the long range aspect to control battles by attacking from far away with few or no minuses vs. them getting hefty penalties. It also allows for simple tactics for when you lose the initiative, you just move away. That's if they don't have a Coilgun too, but maybe even then if they have superior short range weapons.
Early on I joked we should put our heaviest weapons on our tail (as well as turret, of course), but after half a dozen battles, it's not a joke any more. Those range penalties add up really quickly for the other weapons.
OT: We've often ended battles with shields at full (if a bit unbalanced) and minimal hull damage. Maybe one crit suffered by PCs? I don't feel proud of this, as it's a flaw in a much flawed subsystem. I'd rather narrate the battles at this point. I think I've only enjoyed one Starship battle, and even that felt empty after I realized it was impossible for the enemy to hurt us faster than we could restore the shields once he ran out of missiles...and we could've run out of range (or behind terrain on this map), restored, & returned if he had been hitting with those. He'd have to have hit with all the missiles for solid damage or nearly all for close to max damage to have directly won. Yay, us?
The math is off on shunting power to weapons.
Roll: 1, 2, 3, 4, ave. 2.5 (normal)
Roll: 2, 2, 3, 4, ave. 2.75 (or 11 total for a Coilgun)
(A quicker way is to think you get +1 damage, 1/4 the time, so when looking at d12s, it's +1 damage, 1/12 the time for +.083... each die, basically worthless.)
Even with a Coilgun the PC rolls to do +1 damage ave., that is if the gunner hits. That's rather lame vs. getting those shields fixed or balanced, or boosting the crit chances to hit a vital area. So shunting power to weapons is really a "nothing better to do" option. I'd say that the option needs patching since even with d4s it's unattractive, but I don't expect such a major overhaul in the system.
Thank you for correcting my math. I dunno where the hell i was getting my numbers from, not thinking it through i guess. I've been working on a collection of many smaller T1 fighters vs a T5 party. I feel those are working out better than a 1 on 1 ship fight. Less reliant on a hot shot pilot better than the PCs showing up every fight to make things a challenge. Or having an enemy ship always be a class with a turret.