Rifles have a delay on their shots, don't run, left click and expect to pop a sniper shot off, although the reticule doesn't change much while moving, it causes a delay to shots being fired, which will often throw off your accuracy as you're aiming for that one moment, not for when the shot finally does go off. So make sure you have the gun aimed and have waited a short amount of time before letting rip on the enemy (1 second is enough)