This is certainly an interesting idea, not usable in its current form but with some refining can become very helpful. The way I see it, there's two possible directions this can work.
1) Follow the Leader
The current reticle for gun stays linked to the gun but there is a new smaller reticle (a red diamond for example) that is always in the center of your screen and you can look around anywhere you want, the gun's reticle will just follow your reticle as fast as it can. When it reaches the edge of its arc and can't go further, the circle can just change colors to indicate that (let's say red) making it clear that you reached your arc. Also making it easier for gunners to know if they'll have to use stamina to extend further. There's some flaws with this method. As Watchmaker said, it could screw up long range shots as well as throw off your aim when you're firing and turning at the same time. Which is why I would prefer the other option.
2) Gun/View Toggle
You can turn your gun, or you can look around. There's no interaction between the two. I would make it so holding down space allows you to look around while the gun stays in its current position. I'd say holding down because if you're looky looing around and suddenly get attacked in the front, it'd be quicker to release so you'd get back to firing. (Some people might prefer making it a toggle but hey, this is hypothetical! Why not just make it a setting?) I'd say space because you'd have no reason whatsoever to be jumping when you're on a gun, and it's big, easy to reach, so shiny, just begging to be pressed. Why anyone would use this instead of just getting out of the gun is that your gun stays where you left it and doesn't reset to the center. Nice for the heavier guns that take forever to turn.
So that's my two cents, what do you guys think?
