Like how you rotate the gun.
It's weird because no matter which way the gun is already rotated, it rotates in the direction your mouse goes.
I have a basic enough grasp on CFrames but I can't figure out how to do this?
I know this isn't much of a answer, but you do know a version of Phantom Forces was leaked? You can actually see how it works by opening the leaked version in studio. http://puu.sh/kUXKD/e1853423ac.rbxl I'm not that good with CFrame, else I would of explained it better...