What position must the spindle valve be in for firing grenades or line-throwing projectiles?

The correct response is the horizontal (OFF) position for the spindle valve when firing grenades or line-throwing projectiles. In this configuration, the spindle valve is closed, preventing gas flow and ensuring that the ammunition is secured and cannot be fired unintentionally. This safety precaution is essential when handling these types of ammunition, as it minimizes the risk of accidental discharge.

Setting the spindle valve to the horizontal position creates a safe environment for loading, transporting, and preparing the system for eventual use. When ready to fire, operators would typically switch the valve to the vertical position, which allows gas to flow properly and enables the firing mechanism to operate as intended. Understanding this operation is crucial for maintaining safety during training and mission execution.