Well I don't have a third person aim animation, that is missing, should be doable, but will be hard to align.
Or you do a separate first person gun model with the arms attached to it and remove the rest of the body like other games do, with this solution the animation will always line up perfectly, since the arms are glued to the gun.
You do not change the look animation, you change the aim animation. My solution is pretty cool since it works, much cooler than a not working solution. So don't complain, even most AAA games cannot solve this problem and their third and first person animations do not line up perfectly as well. So my solution is even cooler than theirs, since I have the real body in first person that accurately reflects the animations in first and in third person, well except the iron sight aim animation. I could have fixed it, but it turned out to be more complicated since the default engine code did not have something to put the iron sight aim state into a state machine and attach an animation for it.