I thought i might add my own Theorycrafting:

I think AMM ro be the number one choice.  That said, i run destroyer escorts that have lasers with best resolution FCS.  I think lasers offer the second best option specifically because they are long range, can be turreted, and can be used in ship to ship combat.  That said, i put in some CIWS as well.  They are inexpensive and i want to make sure I kill any leakers.  Thus, my designs tend to have Laser, Missile, AND CIWS PD on my escorts, and just CIWS on my other ships.  I focus on Cruise missiles and fighters for killing stuff

Here to confirm that it does happen that way.
Example. 6HS per gauss cannon, 6250 tracking (which is the 10% size) at my tech level

1 gun, size = 6.6
2 gun, size = 13.14 (instead of 13.2 less than 4% difference)
3 gun, size = 19.66 (19.8, 7% more efficient than 3 singles)
4 gun, size = 26.16 (26.4, 9.1% better than 4 singles)

Now lets add some armor (2)

1 = 9.44
2 = 17.63 (18.88, 7% diff)
3 = 25.53 (28.32, 11% diff)
4 = 33.26 (37.76, 13% diff)

Coincidentally, it gets even more effective the more armor you go with. Though I've never actually tried out, heavily armored turrets in game and seen their effectiveness. But as an example
Say your ship is a beam ship with 8 armor all around, and you're packing 8 more on your turrets, the difference between 1 gun, and 4 guns is 17.94 per single gun vs, 13.64 per gun in quads, almost 30% Also larger lasers seem to be more efficient in large turrets than smaller ones... but theres rarely a use for turreted large lasers in the game, maybe 15cm at best for mixed PD/AS weaponry.


Also for those of you who haven't noticed before, in the turret design window it says in the text box, how much HS/gun is getting used, by increasingly armed turrets. Its marked as SPW, to the right of the overall turret size.

Multi-gun mounts get a percentage of their gear tonnage shaved off as an incentive for not simply using a bunch of single mounts. The change log for 5.50 states this is for laser turrets, but I assumed it applied to all turrets (gauss included) and the phrasing was just part of Steve's pro-laser agenda 

Have I been mistaken about this and just never noticed?

There's no reason to make "quad" turrets of any kind (unless you're armoring the turrets).  Four single turrets are the same size as one quad turret.  So I always do single turrets and then I can use 3, 4, 5, or however many to make it fit into my desired tonnage.

possibly yes
but
past tech 6 you can get any opponent who would test you enough
so only recogition point are Invaders
vs them this rule works well as I wrote

it is rather
you can choose any of
2 - on early small designs
3 on mid game meds
4 on big late nasty warships

but in all seriousness, outside of RP reasons and personal rules I dont use gauss early, even if you win the missile->empty magazine phase by taking out maybe 1-2 more missiles per wave then my laser armed Beamhound AMS fighters, they would would mince you in short order from well outside your gauss firing range ( 1 hanger bay for 2 heavy fighters costs the same as a gauss gun space wise)

Yes, that assumes the 1-2 more missiles per wave per 20HS difference isn't decisive though, because if it is it means the fighters very soon will lack a mother-ship and the gauss defense has survived without little or no damage from some stray hits.

It also assume the offensive payload that the gauss defenses protect doesn't have any kind of anti-fighter weapons / sensors.

scratch that, this is when you should consider re rolling the universe and putting the enemy difficulty modifier 50% up

  Loosing is fun! 

I generally play at 150%-250% anyway.