OD looks great..The droopiness is most likely from the lens. The closer you get in distance to an object (measured from the lens) the more the lens has to 'fisheye' to focus. That bends the sides in or out somewhat. You most likely would never notice this on someones face or similiar subject, but on a razor sharp line of a barrel like that it becomes evident.
Some of the best effects shots use a 10mm lens where there's no focal length, everything is in focus, whether it's an inch in front or a mile away. it's great for miniatures, you can have a person moving say, thirty feet behind a model and it'll look like they are right next to it.