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 are way too inaccurate to get precision damage. They just need higher base damage to make up for the charge time and lack of precision damage. Add penetrating rounds or something.