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. At higher altitudes, the air is thinner, meaning there is less atmospheric pressure and fewer air molecules for the projectile to push against as it travels. This reduced air resistance allows the projectile to maintain its velocity for a longer distance compared to firing at lower altitudes where the denser air increases drag, resulting in a shorter range.

Humidity, temperature, and wind speed can also influence how far a projectile travels, but their effects are not as pronounced as altitude. For instance, higher humidity can reduce air density, while temperature can affect the speed of sound in air, but changes in these factors typically do not have the same significant impact on projectile range as altitude does. Wind speed can alter the trajectory of a bullet during flight but does not affect the inherent range capabilities determined by the conditions of altitude.