Master gunnery sergeant
From Halopedia, the Halo wiki
Master Gunnery Sergeant is the ninth and highest enlisted rank of the UNSC Marine Corps and is a staff non-commissioned officer. It is equal in pay grade to Sergeant Major, although the two have different responsibilities. Master Gunnery Sergeant is equal in rank with the Master Chief Petty Officer in the UNSC Navy.
Master Gunnery Sergeants are sometimes referred to by the nickname "Tops", "Master Guns" or "Master Gunny". These nicknames are unacceptable in formal or ceremonial situations, and, at the rank holder's discretion, may also be unacceptable for use by lower-ranking Marines.
Sergeant Major and Master Gunnery Sergeant
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 Sergeants Major have general command responsibility, while a Master Gunnery Sergeant's role involves providing technical military leadership.
Whether this carries over into the UNSC Marine Corps is unconfirmed; however, this seems to be how it works. The Master Gunnery Sergeant depicted in Cairo Station's armory in Halo 2 obviously had the seniority, experience, and clearance required to service and replace John-117's assault armor, fitting the role of a contemporary Marine Corps Gunner.
