I'm hopefully assuming this is a bug and not intended due to the only one I have has Tactical Mag/Extended Mag as perk options...giving it 7 rounds with or without the Extended Mag mod.
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Seven is maximum, use the slot for a major or boss mod
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Backup Mag simply buffs mag size to the maximum allowable for a weapon type. If that weapon is already at max due to an inbuilt mag size perk, backup mag will provide no further benefit.
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I don’t think that’s a bug. I’m pretty sure that’s the maximum amount of ammo the magazine can have on a rapid fire frame.