I'm thinking about a special kind of fire gun use here : the cover fire.  

When doing a cover fire, it's doesn't really matters if you actually fire bullets or not : if several full-auto guns are firing, no one will raise its head anyway.  

So i wonder if it might be useful to have a gun that :  

 1) can fire real rounds -fair enough-, ...  

 2) but also fake rounds that only make the noise/flame/smoke/... of a real round. 

The advantage being that you could have 3 (?4?) times more ammo in a magazine vs real rounds, and even probably a lighter magazine, thus increasing the operational capacity of those guns. 

**Edit** : both magazine would be loaded at once, and round switch would be instantaneous. One way would be to have two triggers. Another rule : When fake ammo runs-out, firing will bring real ones. 

**Edit 2** ?? I wonder if we could agree about : such change wouldn't bring down the reliability of the gun ???

I understand very well that the enemy would soon know about such fake fire : but anyone using such weapon can, still, switch to real rounds at any time and kill an enemy suspecting a fake fire and leaving its cover : sound is still fearsome, the cover is still working. 

Why isn't this kind of gun massively used ? 

(i have no experience in weapons, i might be missing something obvious...)