Armory officers are those Starfleet personnel that man the weapons storage and operations facilities aboard a Federation starship, starbase or outpost. This job is part of the oversight of the operations division.
In the days of the Earth Starfleet, there were no formal security commands set up on some exploration vessels, such as Enterprise NX-01, so the armory officer was often in charge both tactical functions, and those of shipboard security. Many early policies of Starfleet security and tactical weaponry procedures were set in these early missions (such as the establishment of automatic tactical alert protocols coordinated by the ship's computer, devised by Armory Officer Malcolm Reed of Enterprise). (ENT:"Singularity")
The involvement of armory officers in security operations lessened as MACO troops were assigned to Starfleet vessels during the Xindi conflict of the 2150s, in a separate department that would in later years evolve into the modern Starfleet security department.