Which of the following is NOT a factor that affects a burst of fire?

In the context of machine gun employment, the gunner's physical strength is not a direct factor affecting the characteristics of a burst of fire. While a gunner's physical condition can influence their ability to operate the weapon effectively over time, it does not have a significant impact on the mechanics of how the machine gun operates or the effectiveness of a burst.

On the other hand, clarity of the target is crucial for accuracy during firing, as it determines how well the gunner can see and engage the intended target. The gun and ammunition used directly influence the rate of fire, recoil patterns, and overall effectiveness of the burst. Additionally, air and weather conditions can significantly affect the trajectory of projectiles and overall firing conditions, making them important considerations when delivering accurate fire. Thus, the other factors listed play direct roles in shaping the effectiveness and dynamics of a burst of fire, while a gunner's physical strength does not.