I don't know if this is the right section to post it or if it's already known,however,I won this shotgun yesterday with two perks,one of them is supposed to give me bonus ammo directly in my magazine each time I kill an enemy with this gun,it doesn't work at all tho,I'm pretty sure it is a bug
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You have to unlock that upgrade for the weapon. It's just like your subclass skill tree, but for each weapon. (It's the same with armor as well)