It's so simple! You have to be in within the enemy's melee range to get an effective shot, which won't kill high level enemy's.
Snipers win every time because you can stay out of both melee and gun range
Shotguns will only ever be effective in pve if they where made 1/2 shot killers on lvl 30+ enemies.
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Yup, they would have to be borderline over powered to be worth it. Or have double range with a tighter spread (like before they gutted them)