What term would an observer use to initiate a barrage of fire over a period?

The term that an observer would use to initiate a barrage of fire over a period is "Duration."

In military terms, "Duration" specifically refers to a time frame in which artillery or fire support is conducted. When an observer calls for a duration of fire, they are indicating not just the intensity and type of fire needed, but also how long the fire should continue. This helps coordinate the timing and ensures that the fire support aligns with battlefield operations smoothly.

The other terms, such as "Continuous Fire," generally imply sustained fire but do not convey the specific timing or sequence intended by the observer. "Cease Loading" is used to instruct units to stop preparing rounds, while "Request Splash" pertains to a request for confirmation of effect after a round impacts, rather than initiating fire over a set duration. Thus, "Duration" is the precise phrase that accurately captures the intention of sustained fire over a specified timeframe.