The gunner was a warrant officer in charge of the ship’s weaponry.
Originally, a gunner was almost exclusively responsible for the naval artillery, i.e. the ship’s cannons. He would be in charge of maintenance as well as instructing and commanding the crew in their firing. Once ships got larger and carried too many cannons for a single person to command all of them, the guns would be split into groups supervised by different officers during battle. Gunners also eventually took on duties that were originally the Master-at-arms’, i.e. being in charge of a ship’s armory and instructing the crew in the use of small arms (guns and swords).
Aboard the Obra Dinn
During her final voyage, the Obra Dinn had a gunner named Christian Wolff, a man of Austrian origin. He was assisted by a single mate called Olus Wiater, from Poland. Both can be seen handing out weapons to the crew in Soldiers of the Sea, part 7, implying that they are responsible for the ship’s armory, possibly because the Obra Dinn did not have a Master-at-arms. Neither of them survived the trip.