How do I rotate an object like in Phantom Forces?

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...