Wrong answer ! Nope aint buying it, you will not gain 10 fps second shooting at that angle. The answer is gravity , gravity pulls an arrow down as you shoot horizontally when you shoot down gravity doesnt have that same pull cause your already shooting down, theres no arch in the path of the arrow when your shooting down. same with a rifle shooting down a mountain.
Zrexpilot: Wrong answer. Gravity has the same pull on everything no matter which way it is going. Gravity is an acceleration, therefore all objects will feel the same acceleration until they meet terminal velocity. Also there IS "arch" in anything you shoot; rifle, bow, or sling shot. It doesn't matter if you are shooting uphill or downhill, it has a trajectory that is not flat. As soon as a projectile leaves gravity is automatically pulling it off the horizontal path. For this reason bullets and arrows are must be aimed high in order to hit a down range target at the correct height.