(A) British. The chief gunner in charge of ordnance and ammunition (formerly specifically an officer under the Crown). Now historical except as an honorary title conferred on distinguished soldiers, e.g. Master Gunner of the Tower, Master Gunner of St James's Park. (b) British. A warrant officer in the Royal Artillery who has charge of the stores and equipment in a fortified position or other armed place.
master gunner/ˈmɑːstə ˌɡʌnə/ /ˈmastə ˌɡʌnə/
Late 15th century. From master + gunner.