Run a test mission on a dedicated server with debug console enabled (I attached the repro mission I used).
To summarize, setMagazineTurretAmmo has no effect if:
OR
By using the repro mission on a dedicated server, jump in the M4 Scorcher artillery tank as gunner (don't get inside as driver otherwise test will not be good => locality of vehicle goes from server to player computer).
Execute the following command on server and player computer and notice it fails on both cases (ammo count stays at 32 instead of going to 10):
vehicle player setMagazineTurretAmmo ["32rnd_155mm_mo_shells", 10, [0]]
Note: for execution on server you must first run: myv = vehicle player; publicVariableServer "myv"
then
myv setMagazineTurretAmmo ["32rnd_155mm_mo_shells", 10, [0]]
setMagazineTurretAmmo takes local argument just like many other turret commands. you can check turret locality with turretLocal and set ammo from that location or can just spam it on every computer, only one will work.
Also the command is broken, personally I did not want to add it to biki until it is fixed, but someone already did.
I tried what you suggest Kid but it didn't work. I was on dedicated server with another player. I went in driver seat of Scorcher. He went in gunner seat. I executed the command on my computer and on his computer without any effect (I don't remember executing on server side but I don't think that would have worked either). The only way it works is when both turret and vehicle are local, so it will never work when the locality of turret and vehicle is different.
Just checked, and yes you are right this is the case. I will add this broken locality to my ticket too.
By the way, I checked addMagazineTurret under same condition and it worked as intended. setMagazineTurretAmmo has been broken since it was introduced, now we also know how much :)