RE: shootin off a cliff
When shooting at a sharp angle, either up or down, your bullet will tend to strike a bit above your line of sight. This is because the horizontal distance traveled is shorter than it seems. Draw a triangle with a short horizontal at the top, a long vertical at the side and then connect them with a third line. The vertical line represents your position where you shoot from and is the vertical distance from you to your target (top of cliff to ground level at target). The Horizontal line is the actual straight line distance between you and your target (the distance gravity has to work on your bullet) the last (angled line) represents the path the bullet takes from muzzle to target and is what you see when you attempt to judge the distance.
At relatively short ranges this doesn't mean much since many cartridges and loads are very flat for the first couple of hundred yards and the difference in actual bullet travel and the horizontal are not that much. At longer ranges and at extreme angles (>45 deg.) then this may give you a problem. Practice will help and a good balistics program will give you an idea of what you are up against. Good luck.