Shotguns only do their damage up to the end of their normal range. And that has been reduced to melee range, which is about 5 feet. Outside of that, the lead shot pellets are magically transformed into tiny marshmallows which do no damage apart from the sting of being hit by a couple thousand marshmallows.
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Yeah its pretty silly. Its basically turning a shotgun from a firearm into a melee weapon. Its range will be so short that you'd do just as well to turn it around and CLUB your opponent with it.