Fusion rifles don't see much use in PvE because they are out damaged by shotguns most of the time and require more time to shoot. If fusions were given the ability to crit it would reward slower shots for extra damage. (This is only a PvE post, and hope they dont give fusions the ability to crit other players).
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Fusions shots go through enemies. Line em up if you want to take of the weapons abilities. Understand why they don't crit when it can voop a line of trash mobs in one burst.