It's a good thing to remember that buffs and nerfs affect the entire sandbox. So while fusion rifles got a slight nerf, and I do mean slight, they also got a lot of indirect buffs.
This is ignoring kill times in PvP and PvE, that's irrelevant and frankly another subject.
English
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We're nerfing shotguns to make fusion rifles more attractive! But we don't want everyone using the same weapon type, so we need to nerf fusion rifles as well. Only when all special weapons compete with Sidearms will our balancing be complete! Just in case someone's sarcasm detector is broken, I do not speak for Bungie.