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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I use a fusion rifle in pve all the time, I seldom use a shotgun unless I need it specifically. Fusion out range shotguns and are more fun, specially telesto.