1) Also known as "he who bringith the shit upon ye", the master guns usually likes to go around during the day and point out what you or your Marines are doing wrong and/or what you should be doing.
This individual instills a sence of fear in all who are lower ranking to him/her.
2) Marine Corps abbreviation for: Master Gunnery Sergeant (E-9).
LCpl Slacks-a-lot: "Oh shit, turn off that movie! Here comes the master guns!"