Oh, one thing I believe (no hard facts):
If you are grinding for a certain weapon, never hand in one package, grind, hand in one package, etc etc.
Instead, stack it up, to have enough material to fire up a couple of packages (6-7 at least).
Than unlock the first package, check the preview if the gear is available (if not, wait 5min and preview again)...if the gear is in, fire up all the packages as fast as possible.
It seems to me that firing up multiple packages tricks the so-called RNG of bungie. Typically you get no duplicates this way, hence your chances of getting the gear you want drastically increases!
I got Nameless Midnight and the Manannan and the Antiope this way.
Still no luck with better devils...takes forever to stack crucible packages
PS: oh, one last thing in my loot grind believe: the purple fireteam coin that increases the chance of loot. My believe is, that this coins somehow open up different paths in the loot grind. If you never get the gear you want, even after handing in 10 packages at once, try to change your purple coin: if you used it, than don't use it anymore. If you didn't use it, try it again with the coin. For me, this worked.
Could be all just a placebo, as with every believe system.