This mod can be purchased unranked from Baro Ki'Teer for 200,000 Credits 200,000 and 280 Ducats 280. Note however that Baro Ki'Teer's stock changes with each appearance, and may not have this item available at every time.
Works particularly well with the Kuva Hek, as it grants the gun an additional four rounds of ammunition and allows a quick succession of its heavy burst alt-fire shots.
Works also well with the Tigris shotgun series, allowing a quick fire of four rounds within a second and increasing overall DPS due to its fairly quick reload speed.
Primed Ammo Stock was an unintentionally released and briefly replaced all standard copies of Ammo Stock between Update 16.4 (2015-04-23) and Hotfix 16.4.1 (2015-04-23), however aside from its Legendary rarity, the two mods were otherwise identical.