Ammo is stored in various sized ammo cans, this includes the linked 40mm grenades for a MK-19. Mortar rounds are stored in long flat boxes, usually four or five rounds per box. Hand grenades are stored in their own cylindrical container, with the containers stored in wooden crates. 40mm grenades for an M-203 are stored in a similar way.
Keep in mind though, ammo and weapons are not stored together. Weapons are stored in various types of lockable racks. Ammo and other things that go boom are stored in a building called a magazine. Rack magazine stores a different type of ammo/explosive. Gun ammo would be stored in one magazine, morter rounds, grenades and artillery would all be stored separately, depending on what type of projectile it is: High Explosive, white and red phosphorus, and what not are all stored separately too.
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