Hi, my friend and I noticed last night on Xbox one that if we equipped a different special weapon while on a mission, the ammo counter would be set to zero. If we then re-equipped the previous special weapon, it would also have zero ammo. This did not occur in every single instance of equipping different weapons, but happened more often than not.
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if this is when you change it in the middle of the game they did it on purpose to punish you for changing it