Just to clarify before people think they got a “got cha” moment no i pick up 1 gun not my whole squad so again it shouldnt be 900 but hey i aint complaining it a good hidden buff.
This actually happened to me once with the machinegun during the zombie Halloween event (I think it was the RD44). I think it had to do with me shooting a bunch of zombies and then reloading half empty belts, and then replenishing ammo several times and then all of a sudden I would replenish my ammo and it kept going more then then max, up to I think 600 or 900 when I noticed.