Which gun is used on the HH-60 aircraft and can be reloaded midair?

The GAU-18 is the correct choice as it is specifically designed for use on the HH-60 aircraft, which includes the UH-60 Black Hawk and its variants. This weapon is a .50 caliber machine gun that can be mounted on the aircraft, allowing for effective aerial support.

One of the key features of the GAU-18 is its capability for midair reloading, which is advantageous during extended flight operations since it allows for sustained firepower without needing to land for ammunition replenishment. This operational flexibility is crucial in combat situations where continuous aerial support may be required.

In contrast, while the other options listed serve different roles or are utilized on other platforms, they do not possess the same midair reloading capability or are not specifically designed for the HH-60 platform. The GAU-17 is a minigun best known for its high rate of fire and typical use on other aircraft, while the GAU-22 is more modern and used on different platforms, and the MK-19 is a grenade launcher suited for ground operations. Therefore, the GAU-18 stands out as the appropriate and effective weapon for the HH-60 with its unique features and capabilities.