Another note about the Sentry Gun turret design:
The current implementation lacks anticipated modding features: The ability to configure the number of allowable waypoints. By setting this to two waypoints, the rocket would be able to hook out around the end of the landing craft and hit targets, but would not be as OP as say, the blaster launcher. This ability has been implemented in the nightlies, but not yet been merged into the OpenXcom Extended branch. This will happen the next time Yankes synchs these up.
tldr: It will get better, and more useful down the road.