In a way, HZ-12's "extra" is precisely the ability to fire two shots in very quick succession. It's almost a boomstick with extra rounds. And considering a lot of weapons don't have anything "special", I'd say it's already fairly interesting.
I'd go as far as to say that the HZ-12 is one of my favourite gun out of the support's arsenal (and considering almost all of his weapons are effective: that's no easy feat!)
Again, I get your idea and I doubt it would ruin the game or anything. Just, I don't believe it was the idea behind the gun.