Well its potential OPness depends on how much of the functionality of the Crescent Rose you want to incluse. If your just after the scyth/rifle thing then you could use the dwarves thrower as a base to get an uncommon level magic item (using the glance as a scythe for stats):
This scythe has the ammunition property with a normal range of 20 feet and a long range of 60 feet. When you hit with a ranged attack using this weapon, it deals an extra 1d10 damage. At the beginning of your next turn the scythe reloads itself without the need to provide ammunition.
If you want to include the dust stuff as giving mechanical effects that's a bit harder, as I'm not sure whether to count that as a power of the weapon or the ammunition. Fire and Ice powders are easy, they just reskin the damage type, gravity clearly deals force but I'm not sure how to grant the 'increased speed' without getting a bit strong, the electric dust could have the shocking grasp effect on a hit (as well as changing damage type to lightning). If you go the ammo path, I recon the first two are uncommon, the third is very rare and the fourth rare. If you decide to attribute the effects to the item, it's almost an artefact level item (but would need to get some + to truly be that), and hence a little op.
- "the DM is a mystical figure from another plane of reality that can only communicate with us through the magical program known as Skype" - Nasim "Knifey" Val