Nothing. Rerolling would be dumb because it's still a good perk in PvE and against any overshield in PvP.
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It'll one shot a shielded enemy with or without final round. So that doesn't matter. It's a marginally useful perk in PVE. It rarely is the difference between killing and not killing an enemy in PVE. It's absolutely pointless in PVP now, which I am really happy about. Nothing was more frustrating than hearing your opponent burning the first two rounds in their magazine just to get that easy kill.