The factor that has a significant impact on a firearm's maximum projectile range is altitude. Altitude affects the air density, which influences the drag force acting on the projectile. At higher altitudes, the air is less dense, reducing drag and allowing the bullet to travel farther, thus increasing the maximum projectile range. Other factors such as temperature and weather conditions have less direct impact compared to altitude.

Additional factors also affecting projectile range include muzzle velocity (the speed of the bullet when exiting the barrel), barrel length, rifling (which stabilizes the bullet), weather conditions like wind, and the angle at which the firearm is aimed. However, among these, altitude stands out as a significant environmental factor because it alters air density and thus drag force on the projectile.