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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Yeah Telesto with crit damage. There's no way to split it so you can crit shot in PvE and not in PvP and TBH fusion rifles so an insane amount of damage in PvE without crit damage. I used Telesto on the Volundr boss before with a tether oh my it melted him.