Cause: It is normal for the image of a target to be less clear at 500 yards than it is at 100 yards. For example, the image of more distant targets will be degraded by several factors which include dust, haze, and heat distortion (mirage). These will all affect the clarity of the image and, reduce your ability to discern small details at long distances.
Solution:Glassing in calm cool conditions, such as a clear morning, will give you the best long range image. Reducing the magnification of your spotting scope can also reduce these effects, but they are not 100% avoidable. Depending on the color of the target and backdrop, you should not expect to see 30 caliber bullet holes on targets beyond 300 yards during hot or dusty conditions, regardless of the quality and magnification of your spotting scope.