Gunner refers to anyone whose main job is to operate a gun.

• Hand rifles are generally considered to not be guns, and the appropriate term to describe one is rifleman (which is gendre non-specific, -man is the suffix of woman.)
• Heavier rifles which are intended to be used against anything more than a single person can be freely called guns, and the operator can be called a gunner. This can include .50" or greater anti-material rifles; recoilless rifles; or some assault rifles.
• An air gunner operates any of a range of flexibly-mounted machine guns or cannons used in an aircraft.
• Machineguns are typically manned by machinegunners rather than gunners, or they are often referred to by their weapon, i.e. the first machinegunner in a section might be called #1LMG (Light Machine Gun,) the platoon machine gunner might be called 1GPMG (General Purpose Machine Gun,) but these only apply when they are currently carrying the weapon, (if a light machine gunner hands off his weapon to join a recce party, than the person who takes his weapon would then be called #LMG.)
• Those who carry Sub-machine guns or shotguns don't have a specific term to describe them, and are often referred to by their role (MOUT soldier or urban infantry,) or are simply called light infantry.
• Gunner (Gnr) is a military rank in artillery regiments, equivalent to private or pfc. The rank equivalent to corporal is Bombardier.
• In the U.S. Marine Corps, the term gunner is used to refer or address a warrant officer regardless of his or her rank or technical speciality. It is usually considered inappropriate to use the term in formal situations, or where there is a lack of rapport or familiarity with those dealing with a warrant officer.

The word has also several uses in non-military contexts.

• A gunner in educational colloquial terminology, especially in medical-related education circles, is a person who is competitive, overly-ambitious and substantially exceeds minimum requirements. A gunner will compromise his/her peer relationships and/or reputation among peers in order to obtain recognition and praise from his/her superiors.
• Gunner is also the name of a non-alcoholic cocktail enjoyed by British expats in Hong Kong and other parts of the Far East and India. It consists of equal parts ginger ale and ginger beer with a dash of Angostura bitters. It is famed for its refreshing qualities and is drunk in large quantities in the summer.
• The Gunners is a nickname for Arsenal FC, a London football team (which was originally founded by workers at the Woolwich Arsenal). It is also the nickname for the Edinburgh rugby team.