Gunnery Sergeant is the seventh enlisted rank of the UNSC Marine Corps, and is a staff non-commissioned officer rank. It is equivalent to the rank of Chief Petty Officer in the UNSC Navy, Sergeant First Class in the UNSC Army, and Master Sergeant in the Air Force. Gunnery Sergeant ranks above Staff Sergeant and below Master Sergeant.
Gunnery Sergeants are commonly referred to by the informal abbreviation, "Gunny". This nickname is generally acceptable for use in all situations except formal and ceremonial settings. Use of the term by lower-ranking personnel, however, remains at the Gunnery Sergeant's discretion.
In the United States Marine Corps, eligible Gunnery Sergeants indicate whether they wish to be considered for promotion to Master Sergeant or First Sergeant, and thus, whether they enter the promotional track for Master Gunnery Sergeant or Sergeant Major. Once promoted, the promotional track is permanent; lateral movement between the two programs is not possible.
One of the major differences between the two ranks is that First Sergeants/Sergeant Majors have a general command status, while a Master Sergeant/Master Gunnery Sergeant's role is as a provider of technical military leadership.
Whether this remains the same for the UNSC Marine Corps is unconfirmed, although, this would appear to be true as indicated by events depicted in Halo 2 in Cairo Station's armory. The Master Gunnery Sergeant there seemed to be a systems tech of some sort who obviously had the necessary clearance and expertise to service, maintain, and replace MJOLNIR Assault Armor, filling the contemporary role of a Marine Corps Gunner.
In the USMC, a Gunnery Sergeant typically serves as a supply and operations liaison at the company level. They serve as assistants and advisers for Lieutenants. It is highly probable the same responsibilities carry over to the UNSC Gunnery Sergeants, but this is unconfirmed as of yet.