Back and tip of the cannon are both a bit heavy on black. The lighting looks inconsistent too, see how the shadow runs along the top of the cannon until it reaches the tip where it's light? Similarly there is a light section at the back of the cannon running the length until the tip again where shadow from below takes over. There is also a halo around the cannon (likely a result of shadows/highlights or just general D&B - most prominent at the back) which needs addressing. I would probably bring the clouds up a touch and just go with a standard vignette too, as it looks a little overpowering in the two top corners.