I'm pretty sure they do disappear already.
Spoiler
(http://i58.tinypic.com/14uanfb.png)
The Hephaestus AG in the center has depleted it's ammo, and there's no arc.
EDIT:
By the way, a bug (I think) - when a weapon has recoil in the code, all the barrels will go back to the farthest point of recoil when it's out of ammo. And they will stay there, and when the gun is fired and the weapon fire each barrel individually, both barrels recoil (even farther back than normal because the barrels reset to the farthest point) at the same time.