Of course it would need some kind of ammo limitation, I don't think an infinite ammo beam weapon would be balanced.
Continuing the theme of using lenses to focus the light, and knowing how, when you use lenses that maybe aren't the best quality to focus light it makes a burn mark on the lens, I would suggest that the ammo be sort of a lifetime per lens deal, and when you break, or burn up, a lens, it signifies you running out of ammo. The animation would involve mechanisms pulling a black smoking lens out of the machinery and replacing it with a shiny new one.
This would be a very interesting heavy weapon. I like the idea of it having flechette logically. Lasers are often used to pop balloons, so having fire and balloon popping would work well together logically.
Would be interesting, though not necessarily feasible, to let gunners use scenery to help them shoot. I don't know what the arcs would be, but if you could shoot down at water in one of the water maps and have the beam reflect back up towards the enemy it would add in some interesting tactics. It could be a heavy gun with better downward arcs, unlike the lumberjack which has good upwards arcs.