ok here some facts:
a heavy mech can easy hold 10k ammo. a turret is a structure that has no other purpose then shooting and it is not ment to walk around. taking this in consideration a turret should have a way higher ammo stash then any bot that walks around the field.
so lets say with reason a turret can hold 10x ammo a bot can (even factor 100 would be still ok for a statonary building but lets stick with factor 10). that would be 100k ammo.
taking a 5sec rof at avarage and 2 guns (what turrets usualy have):
100.000 x 5s = 500.000s totl operation time with one gun
500.000s / 60 / 60 / 2 = 69,44h = 2,89d of continuis firering. (/60 mins; /60 h; 2 guns)
well not much fun waiting for the turret to run out of ammo.
ok lets look at it from another side:
structures are already expensive. yes turrets are realtive cheep but the costs are in the supporting structures. all that costs a hell lot money or resources.
so why have to bind hundret of thouthands of ammunition just that the turrets have something to feed when the time arises. some turrets have falloff and as mentioned above players will take advantage of that. just as some ppl kill observer by standing at max range and wait till the observer runs out of cap while doing crap dmg because it is at the last edge of his falloff.
first thing: if players can abuse it then its a bad idea.
second thing: will it realy bring another gameplay and will it make the game better, or will it just increase the grind and will be even more work for no real benefit in the end. later will be the case as far as my opinion goes.
using a supply building will even make a already complex structure system more complex and ppl are already complaining about the network comlexity and that tey loose the overfew of their network.
also if the turrets have a basic ammo stash (just as shown above) then some pll may skip the ammo building and use the extra connection for even more dps/range/whatever.
one more thought:
reactors are producing energy out of thin air. whel then turrets can produce ammo on their own ^^...
in generel the idea of turrets usesing ammo sounds logical. but there are other logical things as well. just like the ammo stash i pointed out above. in general this will bring no new gameplay to the game. yes i like complex thinks but this is just adding more complexity to no result. and with that it is a bad idea.