This is very interesting. I would be hesitant about the increased damage at long range thing, though. If you are aiming for some realism, then this part should be inverted. Light emitted from a single point dissipates as it travels. It should theoretically do less damage the further away you are.
This would have to be a raycast weapon. The concepts of this gun mean that giving it projectile rounds could cause major lag.
Personally, I think I would do a light-ray weapon like this as a kind of experimental sniping gun. It has a very very quick reload, short of instantaneous, and it's damage gets larger the less ammo is in the clip. Each shot does fire damage without a chance to apply fire stacks, and it fires very fast.
I don't know how practical my idea is, though. I think you guys' suggestions fit the current meta better. I thought that the damage relevant to ammo remaining would be an interesting tactic to consider. I reasoned that the focusing lenses would adjust as the gun fired, increasing the intensity of the beam. The damage would have to be independent of clip adjustments made by ammo types though. Again, this is just an idea. To be honest I don't think that this could be implemented.
I like the whole concept though. I hope Muse considers all the things in this thread because they're very well thought-out and reasonable. Much moreso than what I thought up :p