Master Gunnery Sergeant
From Halopedia, the Halo wiki
Master Gunnery Sergeant, abbreviated to MGSGT, is an Enlistment rank of E-9 in the United Nations Space Command Unified Ground Command (UNICOM) rank structure. It is preceded by Master Sergeant, it is equal in grade to the command career track Sergeant Major.
- Halo: Official Spartan Field Manual - PART 8: UNSC ORGANISATION, UNICOM Ranks, page 142-143