Armory officers are those Starfleet personnel that command 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 exploration vessels (such as the NX-Class Enterprise) so the armory officer was often in charge of both tactical station 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). The terms "armory officer" and "tactical officer" were used interchangably, as the two jobs were occupied by the same man. (ENT:"Singularity")
The involvement of armory personnel in security operations lessened as MACO troops were assigned to Starfleet vessels during and after the Xindi conflict of the 2150s. Some functions of this department would in later years evolve into the modern Starfleet Security department, where many of the same roles played in the Earth Starfleet would be assigned to the position of security chief -- which is sometimes occupied as the same individual who is chief tactical officer.