The bulk of my discussion is in the title. I feel that in respect to their semblance's, and general personality, their weapons are more suited for the other character.

Let's start with Ren. His semblance, Tranquility, allows him to mask his negative emotions, and make him undetectable by Grimm. I don't believe it makes him invisible to them, but regardless, this would give him a distinct advantage as a sniper for two reasons. One, the lack of sight by the Grimm would let him keep his Semblance active and choose different locations for more tactical sniping. Two, his Semblance calms his emotions, which is an obvious accuracy advantage. If the Grimm did close in on him, Ren would be perfectly capable of using crescent rose's recoil fighting style, and honestly, it kinda suits his fighting style there as well. His Semblance, does not favor the up close smg nature of StormFlower

On to Ruby. Her Semblance, Petal Burst, allows her to move at incredibly fast speeds, either physically, or as a stream of rose petals... I think. Either way, her Semblance is most useful at providing her with mid to close range advantages, i.e. positioning, avoiding damage, momentum based movement. The latter of which she of course does with Crescent Rose, but the synergy of her fighting style and Semblance do not match a sniper. It would be greatly improved with StormFlower, imagine her fighting like Levi from Attack on Titan, using her mobility to create whirlwind strikes that would end up being more devastating. When it comes to the smg portion, I picture her dashing around like Tracer from Overwatch, zigzag and shoot, zigzag and shoot.

Anyway, what weapons do you think belong with someone else more?