Final answer:
The blank in the student's question should be filled with "breechblock key" or "firing mechanism", which are essential components of artillery operations. The breechblock must be resting on the breech stool for safety before these components are removed.
Step-by-step explanation:
The student's question pertains to a specific part of artillery operations, namely, the process involved in handling a breechblock of a gun. The sentence should be completed with "breechblock key" or "firing mechanism", depending on the exact context. The breechblock is a critical component of breech-loading firearms, which seals the gun barrel's rear opening, preventing the escape of gases when the gun is fired. It must be safely and securely resting on what is known as a breech stool, which supports it when not engaged. This ensures that the mechanism is not accidentally discharged or damaged when the key or firing mechanism is removed.