So, it's not fully clear yet: due to the aforementioned mechanics of gauss cannons, would quad gauss turrets be the perfect point-defense weapon (perhaps excluding AMMs)? Or at least considerably better than quad lasers?
A quadd gauss cannon with a decent ROF is an excellent pd weapon. It has two drawbacks of being more massive than the typical laser or meson used in point defense (10-15cm weapons) and it's short range. It has two major benifits. It does not require a power plant which is nice for removing the explosion chance, and it can have a ROF of double any other weapon in the game.
A quadd gauss cannon turret with each cannon having a rof of 6 would get a total of 24 shots to hit incomming missiles. It could only be fired at 1 salvo, and at best will only knock down 24 missiles, but that is a lot of incomming missiles. It would also mass somewhere in the 35 hull space range which is a huge chunk of a ship. A quadd 15cm laser with capaciter 6 could also fire every 5 seconds but would only get 4 shots. It would also be 23 hull spaces while a 10cm would be 17 hull spaces. Both lasers would drastically outrange the gauss cannon so it is really a choice you are making of do you want a duall purpose weapon, or is it only going to be used for stopping missile attacks. If you switch to mesons from lasers you will still have about 3 times to range of the gauss cannon, and it is also a very effective anti ship weapon as well. It all depends on what you are doing with the design. The other advantage that lasers have over gauss cannon only applies if you are shooting at armoured missiles. In that case the extra damage they do is more likely to shoot down an incomming missile. Mesons just flat ignore armour on the missiles so if they hit it will kill a missile.
I normally do not put a quadd gauss turret on anything except a capital ship, or a dedicated escort. I also pair it up with it's own pd fire control so I can engage the maximum number of salvo's. with the other turrets I will put them in twin or tripple mounts on smaller warships, sometimes as the only weapons and have about 6-8 shots (2 turrets) per fire control.
Each salvo is a seperate target for fire control. Each missile in the salvo is a seperate entity with it's own hit points/armour. One shot can only kill one missile in one salvo. One fire control can fire at only one salvo, but it can fire as many shots as the weapons assigned to it have.
Brian