“Ultimately, the farther a sniper can be from his target and still remain accurate, the more effective he is and the less likely he is to be discovered. Using a 7.62mm round, snipers can shoot nearly silently as long as they're shooting from over 600 meters. A bullet leaves the rifle barrel faster than the speed of sound. The cracking sound a bullet makes is a tiny sonic boom. Even if a target doesn't hear the rifle shot, he will hear the bullet fly by. But the drag created by wind resistance on a 7.62mm round as it travels through the air slows the bullet down to sub-sonic speeds at around 600 meters. So at ranges over 600 meters, the bullet no longer makes that distinct cracking sound. Army Ranger Sniper tells us, "If you're shooting at a target 800 or 1,000 meters out, you could be shooting at that person all day long and they don't even know they are being shot at."
(SOURCE: How Military Snipers Work by Robert Valdes)
In the picture is UDMC's S7-DGIS in caliber 7.62 x 51mm NATO.