Which of the following factors does NOT affect when a periodic inspection might become due?

The timing of a periodic inspection for aircraft armament systems is influenced by various operational and usage metrics, which include the number of flying hours accumulated, the number of rounds fired by a gun system, and the expiration of calendar days based on established maintenance schedules. Each of these factors directly relates to the wear and tear on the systems, ensuring they are maintained adequately to adhere to safety and operational standards.

Weather conditions, however, generally do not play a role in determining when a periodic inspection is due. While adverse weather can impact flight operations and the performance of systems, it does not inherently affect the maintenance schedule set through hours of operation, rounds fired, or specific calendar timeframes. Therefore, weather conditions are not a factor that directs when an inspection is required, making it the correct answer for the question.