In Slammer Up for example there are 3 mags of 2000Rnd_65x39_Belt, lets call then mag#1, mag#2, mag#3
Initially magazineTurretAmmo reports ammo in mag#1 and setMagazineTurretAmmo will also set new ammo count in mag#1
But when the mag is finished and new mag#2 is reloaded, magazineTurretAmmo is still reporting ammo for mag#1 even though it is discarded. If you try to set ammo again setMagazineTurretAmmo sets it for mag#3 instead and magazineTurretAmmo starts reporting ammo for mag#3 as well.
mag#2 that is currently loaded therefore cannot be read or altered in any way.