Ruby doesn't use her gun's recoil to move. She uses it for some extra kick, that's all. The recoil is just [i]added[/i] momentum, but it's her semblance that makes her move really fast for the most part. I would assume that they also just bent the laws of physics a little, and that in "reality" her gun's recoil wouldn't have nearly as much effect as they suggest it does. However, looking at the Red Trailer, we can guess that Ruby is using some big f***ing bullets judging by the holes they put into the beowolves. I would guess some sort of caliber like .50 BMG or very close, like one or two steps above. There are calibers above .50 that people can shoot without being knocked on their ass, as long as they know what they're doing.

But yeah, it's not like Ruby uses her gun to go from a dead stop to lightning speed, normally we see her use it to help her change direction, make her weapon hit harder when she swings her, or to help her move even faster than she already is once she's started moving using her semblance or something.