In the weapon blue print, I attached a camera slightly above the weapon. I have its mesh drawing enabled, so with the weapon equipped, I can see the camera following my weapons rotation.
When the aim action is triggered, I am successfully switched to that camera. This I can see by the narrower FOV. However, now my vertical mouse inputs are ignored. I can still turn left and right. When I exit the aiming process however, I will have moved vertically.
In other words: I pick up the gun, I right click, I move my mouse up, my screen doesnt change, I release right click, and I return to my normal first person view, looking up in the sky.
However, this solution contains a problem: it change pawn pitch wholly. So, if your FPS character has body or you’re going to add a third person character view with first person aiming, you need to take it into consideration.