Which factor significantly impacts a firearm's maximum projectile range?

Altitude significantly impacts a firearm's maximum projectile range due to the effects of air density on bullet trajectory. At higher altitudes, the air is thinner, meaning there is less aerodynamic drag acting on the projectile as it travels. This reduction in drag allows bullets to travel further compared to sea level, where the air is denser. The thinner air at high altitudes can enhance the velocity and distance a bullet can achieve.

In contrast, factors such as humidity, wind speed, and temperature do influence shooting conditions, but their effects on maximum projectile range are not as pronounced as altitude. For instance, while wind can push a bullet off course and humidity can slightly affect air density, neither changes the fundamental aerodynamic properties as dramatically as altitude does. Temperature can affect air density as well, but its impact is more variable compared to the consistent reduction in drag experienced at higher elevations.