If drops were guaranteed then everyone would have all raid gear/weapons probably within 3 weeks and then there would be no reason to raid anymore. I do think that the drop rates should be increased though.
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"Everyone would have all raid gear/weapons probably within 3 weeks" Who said they have to drop at 320 first try?
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I didn't
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So therefore, you still have an incentive to run the raid as you would try to get full 320 gear and weapons.
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Not to mention getting the perks you want. different weapon type reload speeds on gauntlets, different ammo / armor / recovery increases on chest / boots.