Honestly I could somewhat understand all the other gear you find being somewhat randomized, but end game gear should have set in stone values.
As it is I have read of people that ran KFNM for weeks and couldn't get beyond 305-308 with a few 309's spread throughout. Now some people are having the same issues trying to get to 320 light.
I like there are actually separate versions of gear for NM and HM, but they honestly shouldn't have random values, it makes the game much more grind heavy than the forever 29 days.
Edit: Another thought would make it so it scales to your level. you could get up to the level and 5 levels higher.
For example: You have a character that is at 300 in the normal mode, they would get 300-305 gear. Getting to 305 means they would get 305-310. Once they hit 310 however they would have to move on to the hard raid to get higher gear.
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All normal raid gear should be 310 with random stats and perks. All hard raid gear should be 320 with random stats and perks.