It only does one hit body shot kills if the gun has enough impact by default to make this possible with final round. And it's not too op, it only works with one shot and you burn through your reserve ammo really fast.
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For my sniper, without any ammo perks it can get 16 rounds. With perks that allows you to take more special ammo, I can carry 21. It's mag is 3 so I have 7 powerful shot. Not considered picking ammo from special ammo boxes.