Further thoughts:

• It looks like you took the hex-limiting arc off intentionally. That's an odd choice for a primary DPS weapon with a good range, as it's one of the balance factors to prevent people making gigantic armoured bricks. It also has a 360 degree arc, but it does comparable to railgun DPS (or more vs supports) at greater than missile range - is this really what you want? I suspect it's flat out superior to railguns right now in all cases. I would design a ship with a few of these barely poking out of the back end and a load of armour all around, and they'd still get 360 coverage. Seems a bit strong. Observe: http://puu.sh/gDQiY/efc844a22d.jpg
  
• Color of system is same as vanilla railguns - can't tell apart on ship design screen. Perhaps make them more orangey?
  
• You've overwritten combat.as, causing incompatibility with any other mod that does that, just to make the -visual- radius of the explosions smaller (the actual damage radius is still the same). You don't need to do this either - you can just make something like a "flakcannon.as" and put AreaExplDamage1 in there. I'd recommend renaming it to SmallExplosionAreaExplDamage or something else as well. Then you can call it from your effect by using flakcannon::SmallExplosionAreaDamage().
  
• Likewise, you don't need to and should not override weapon.txt in effects. Prefer adding a new file unless you actually need to change the base effects, and use a namespace if the effector doesn't pick it up (but the game is usually pretty good at grabbing every possible effect it can - if the txt file is in the right place, the mod editor should see it after a game restart).
  
• It outranges missile systems, so you can take out creep camps without any particular problem
  
• It looks very pretty, especially with a lot of them firing on a big fleet. http://puu.sh/gDQEJ/e2c02b76eb.jpg - Flakhemoth taking out a Thereus camp despite 3:1 strength disparity - though it didn't even have to close to within range...
  
• With the massive spread, the DPS against single large targets is pretty low - this seems fine, the actual damage of it is probably about right after some battlefield testing, maybe even slightly low. You might want to tighten the spread just a little (perhaps 0.18?), even if you do drop the range.
  
• Performance at optimal range: http://puu.sh/gDQQu/925b60af86.jpg - acceptable :)
  
• Do you know what VariableTimed(Reload, 0) does in the Activation part of the effector? I don't, but I'd like to. I should probably be using this for point defense guns so they don't fire in bursts... They were my idea for solving the same problem, though I think they're a bit less well thought out than yours for that. They tend to be rather good vs shields as well as I put DPS and accuracy up in exchange for low armour penetration... Better vs 100 * size 32 ships, wheras yours is better vs 3200 * size 1 ships.

My overall recommendations:
Drop the range, it just can't be allowed to outrange missiles. Arguably missiles are actually quite short range, with only 150 units more than railguns to start with, but that isn't a problem you should fix with such a specific mod.
Remove the 360 degree arc and the ignoring of hex limits - it can keep a really high arc, maybe 5 or something, and then people can make a decision - do I stick this gun out on a pylon where it will easily be shot off but it can fire in any direction, or do I sink it into an armoured recess so it can only fire forwards? In my opinion those are good decisions, and it gives flanking a strong tactical role. Missiles and rockets get away with this in-universe because they're guided, and out-universe in the game design space because they cost SO MUCH supply, and are thus very expensive to use for any great length of time.

It's a great idea though, I like the style of it and the intent, so please take this big post as advice and balance suggestions rather than harsh criticism. Overall it's good, and with some tweaking should be really nice.

If you happen to be good at them, giving it a new custom particle effect (I suggest orange here too, as it happens) would a) differentiate it from torpedos, and b) let you get away with using the default area damage script because you could just make the effect smaller, I think.