I would probably give it a Cooldown and duration or give it a short "wind up" time of a second or so so that you can't just suddenly stop and get a big burst of fire rate, but otherwise I think the item concept is cool. It would work well for heroes who have very long reload speeds (like Mcginnis). or have long range such as Vindicta.
I would also disallow it's use while flying (meaning you have to be stationary and on the ground to benefit from it) since otherwise it does probably too much to cover one of the weaknesses of flight (being that you are kind of stuck outside of cover while you might need to be reloading).