Over water (completely flat) you have a maximum sight-line of 3.1 miles due to the curve of the earth. Not only is this shot unlikely to be repeatable, but it is downright impossible without a forward observer.

The current record for the farthest sniper shot resulting in a kill is just under 2.2 miles for reference. With the power requirements to get a bullet out that far, all of the longest sniper shots are using effectively anti-material rifles (designed to take out a car engine, not a person).