applying a double mag to a gun followed by a spare mag will produce a mag size of say 30:15 (these values will only be reflected upon full reload)
however removing the double mag will leave the gun with 30 :15 loaded. instead of the new correct size of 15:15. (reloading fixes this)
there are number of ways of solving this but the most realistic way is to cause any modifications to mag size to empty the gun of ammo (except in the case of spare magazine)
note: i don’t know if this error is gun specific or accessory specific, i imagine it isn’t hence why i have posted.
edit: forgot to add these: