.308 Winchester
First things first, the .308 is by far the most popular sniper round. Despite the constant emergence of “new and improved” 30-caliber rounds, they will always be measured against the .308 Winchester.
The fact is, it’s very hard to improve on this cartridge. The ballistics profile of this round hits the sweet spot, traveling long distances with accuracy and consistency, and it even behaves predictably in the wind.
There are hotter and heavier rounds out there. Certainly, there are trendier ones. The generations of military field-testing behind the .308 makes it unique. The US Army caps the recommended range of the .308 at 1,000 meters.
You could certainly hit a target at over that maximum reliably with the .308, however, if you were so inclined.
Photo credit: Credit: JHobb, wikimedia