If an observer wants HE and then WP, what term would they use in the call for fire?

When an observer specifies a need for high explosive (HE) rounds followed by white phosphorus (WP), they are indicating that the first round should be HE and that the second round should be WP. The term "followed by" is crucial in this scenario as it clearly communicates the desired sequence of munitions to the artillery unit. It signifies that the observer is requesting one type of round to be fired first and then another different type afterward. This distinction is important for controlling fire support effectively, as different munitions serve different tactical purposes.

The other choices do not specifically convey the sequential nature of the request. The term "ammunition" refers more generally to the types of munitions but doesn’t denote the order. "Volume of fire" focuses on the quantity and rate of fire rather than the type and sequence of projectiles. "Projectile" is a broad term referring to the fired munitions without addressing the sequence or type clearly. Therefore, "followed by" is the most precise and contextually correct term to denote the required sequence in the call for fire.