Even if you can make fake ammo as cheap as real, while still reliable in weapon and making realistic sounds and you can change fake and real on whim (4 x unrealistic but play with it)
THEN it can help somehow in only few and very Hollywood scenarios, where YOU with lot of units around try to catch ME (with max 2-3 friends in small compact group), on place with just one opening and WE will have nearly no guns and/or ammo. And YOU fire your fake covering fire with occasional real bullet in it, so I would not dare go out. You need suppress my fire for long enought to your forces can reach and neutralize me.
But in any little more realistic scenario you will have big problem: ME and my (possible) friends.
- You are afraid of us (otherwise you will just came clear and plain) and we know it (same reason)
- we can fight somehow and we will.
- if there is only one opening between us, maybe I will be scared to show myself there, while I hear fire from outside, but even so, I will soon realize, that there is nearly no bullet hitting it, so your cover fire is (who know why) concentrated somewhere else. Time for me to try the oldest trick - helmet on a stick
- wow, lucky - helmet really not hit, next time I will try with my weapon and shoot at anything I could see on fast glance - either some soldier running toward me, or you, as your weapon makes some visible flashing effects. Then I fast hide again.
Now you have to spot me on the flash, your gun blasting uselessly and change the ammo to hit me. Would you just wait silently with finger at trigger, you would see and hear more, I will not see your position such easily and I may be less carefull, no bonus for wasting fake ammo, but now you have really hot barrel (as fake make at least as much of heat), with your position nice marked, your gun less acurate (long fire heats the barrel and sustained fire needs to change the barrel sooner or later (in term of minutes)) so you are at disadvatage.
The more, if I am not alone, or I have more windows/holes for use, so I can suppose, that if no sream of bullets goes around me, you are just covering other spot, where I could be/I was/my friend is and I will use the windows of no bullets to send as much bullets your way, as I can.
And I use all real bullets.
If there are like 4 or more automatic weapons on your side and 4 or more any weapons on our side on different spots, I have none real eveidence, WHERE are you shooting with which gun and so I suppose, that all the fire and bullets goes somewhere else and I can shoot safely until some bullets came hit my area near me, before I need to take cover.
So basically if you face few opponents, any of your fake rounds are basically wasted and only the real one counts.
The fakes cannot even hit by luck, or make some debris scracth me, or cover my vision.
(but you have to pay for the fake anyway and carry them and they would heat your weapon so on long fire even consisting of empty bullets, you are forced to change barrel or wait, it it cold a little now and then. And all those fakes makes harder to you spot me (your enemy) and wears your stamina with recoils (hard enought to cycle your weapon reliably).
In any larger encounter (4+ vs 4+) your fakes gave advatage to OUR side and disadvatage to YOUR side.
If you want mixed belts, than tracer helps you see, what point you are exactly hitting, and some incidentary could make fire on my side, some explosive can hit somebody just around corner, or at least scratch his face with fragments, occasional AP would take down some hardened point on our side, that could make some sense sometimes, but empty are just empty and do bad only to your side.
well one use of empty is possible - to panic unarmed civilians without big casaulties, but even so, it is bette to just shoot over their heads to anything durable and give it bettere realistic feel, when small debriss are flying the air and brick powder is on theirs heads ... and if they went to desperately attack you, then you have something to deefend yourself anyway.