An armory officer holds responsibility for issuing, maintaining, storing and regaining weaponry in the armory. In Federation Starfleet, this person usually is a member of the operations division aboard a starship, starbase, outpost or other Federation area.
Aboard Earth Starfleet ships, the armory officer held both tactical and security duties. He or she was sometimes called the "tactical officer" as well. (ENT: "Singularity") When Starfleet assigned MACOs to starships in later years, armory officer duties lessened slightly. (ENT: "The Expanse", et al)
The Earth Starfleet armory officer position evolved in the Federation Starfleet to two separate positions: chief tactical officer and chief of security. These positions were often combined into one. (TNG: "Encounter at Farpoint", et al)