Sergeant major

Sergeant Major is the ninth and highest enlisted rank of the UNSC Marine Corps and is a non-commissioned officer. It is equal in pay grade to Master Gunnery Sergeant, although the two have different responsibilities. Sergeant Major is equal to the rank of Master Chief Petty Officer in the UNSC Navy.

Sergeant Major vs. Master Gunnery Sergeant
In their annual performance evaluations, eligible Gunnery Sergeant's 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 Sergeant Majors have a general command status, while a Master Gunnery Sergeant's role involves providing technical military leadership.

Trivia
Sergeant Major Avery Johnson, is practically the same rank as the Master Chief. Johnson, however, is a member of the UNSC Marine Corp. and Master Chief is a member of the UNSC Navy.