 The ship mounted four 45-calibre 4.7-inch Mk IX guns in single mounts designated 'A', 'B', 'X' and 'Y' from front to rear. For anti-aircraft (AA) defence, Duncan had a single 12-pounder AA gun between her funnels and two quadruple Mark I mounts for the QF 0.5-inch Vickers Mark III machine guns mounted on the sides of her bridge. She was fitted with two above-water quadruple torpedo tube mounts for 21-inch torpedoes. One depth charge rail and two throwers were fitted; 20 depth charges were originally carried, but this increased to 35 shortly after the war began. In 1936, the 12-pounder was replaced by two QF 2-pounder Mk II AA guns. Sometime after the Dunkirk evacuation, the ship's rear torpedo tube mount was removed and replaced by a 12-pounder AA gun and the quadruple 0.5-inch machine guns were replaced by 20-millimetre (0.79 in) Oerlikon AA guns.
