Let's see if I can explain this quickly (I'm in the middle of a honey-do home project
)...
The biggest thing that affects aiming point of impact is gravity, and its corresponding affect on the projectile you're flinging at your target. We aim at a certain point, knowing that the projectile will begin on an upward arc to its target before gravity starts acting on it and begins to pull that arc downward. Hopefully, if we sighted in on the target correctly, that arc intersects perfectly with out target.
When you range an item at ... say, 40 yards -- but it's 40 degrees below your level -- gravity will have less time to act on your projectile (I keep using this descriptive noun because it doesn't matter if it's a bullet or an arrow). The true distance you would shoot for would be closer to like 34 yards, factoring in the lesser time gravity has to act. I'm sure there's MUCH less variance to take into account for gun hunting than in archery, but that's pretty much how it works.
Some math whiz may come on here later and break down everything I just related using pythagorean theorums or something
, but them's the basics! Bottom line? Practice, practice, practice. Know how your weapon performs in similar situations
before you're faced with a real-life scenario.
God bless, and good hunting!