According to Wikipedia, "The post of master–at–arms was introduced to the Royal Navy during the reign of King Charles I; their original duties were to be responsible for the ship's small arms and edged weapons, and to drill the ship's company in their use. This was not an onerous task, and masters–at–arms came to be made responsible for 'regulating duties'; their role as weapons instructors was eventually taken over by the chief gunner."
In the US Navy (again according to Wikipedia), "the master–at–arms rating is responsible for law enforcement, investigations, K–9 [police dogs], expeditionary, and anti–terrorism/force protection operations. It is one of the oldest ratings in the United States Navy, having been recognized since the Navy's inception."
© Haydn Thompson 2023