Changing the weight of your bullet changes its trajectory. Trajectory is the path a bullet takes during flight. Several factors affect this path: gravity, air resistance, speed and weight of the bullet. Gravity pulls the bullet down as it travels forward which results in a downward curved flight path. The air provides resistance that slows the flight of the bullet.
Of course, a bullet travels much too quickly for you to be able to actually see its trajectory. You can see an example of a trajectory though by simply throwing a ball in the air and watching it come down.
The trajectory of a bullet is slightly curved. So, if you sight in your firearm to hit a target at 200 yards, you will hit your target high at 100 yards and low at 300 yards.
Since the trajectory of a bullet depends in part on its weight, you must carefully consider your weight choice. For example, a .30-06 Springfield cartridge with a 180-grain bullet has a different flight pattern than the same cartridge with a 150-grain bullet. Once you make your selection, practice sighting in your rifle using that ammunition before you head into the field.