Rotate to angle of camera? [closed]

I don't understand the camera very much, so how would I make a part rotate to match the angle of the camera? (On the Y axis, so if you look up, the part angles upwards. Look down, and the part turns downward.)

What the point in this?

I made this gun, the camera is set to LockedFirstPerson, and I'd like for the players arms to move up and down with the camera. I'm not sure how to do this. I think it may have something to do with CoordinateFrame, but as I said, I know almost nothing about how the Camera works.