I was with the same problem. For some reason the forward vector is not get the correct direction of the x axis. So i did it by other way.
I Was trying to get the forward vector of my FirePoint of my Barrel. I just put another Scene component at the Pivot:
So when the Turret Moves in the YAW Axis, all the child Scene Component of Barrel moves togheter and everthing is precisely telling me the start point (BarrelBase) and the end point (FirePoint) of my correct forward vector.
To generate the correctly Forward Vector just subtract BarrelBase from FirePoint. Just That.
I Try a lot of other solutions but only that solve my problem. If SomeOne has another Idea to solve it better, please it will be Welcome.