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 don't think fusions or shotguns should be able to crit. I like shotguns being high white damage at a short range instantly and fusions being high white damage at a medium range with delay The problem is shotguns crit, can be used too far away, and shouldn't be getting a fire rate increase with full auto. Having the exotic merciless which is killclip combined with a full auto that drastically increases rate of fire... yet it can't even out damage a growing wave of legendary shotguns just shows that Bungie has lost the damn plot.