The Bag of Holding is an item from Heretic. It gives a small amount of every kind of ammo except mace spheres, and doubles the player's ammo carrying capacity, just like Doom's backpack.

Heretic items
Health	Crystal Vial | Quartz Flask | Mystic Urn
Armor	Silver Shield | Enchanted Shield
Items	Crystal Vial | Map Scroll | Bag of Holding | Ammo
Artifacts	Chaos Device | Morph Ovum | Mystic Urn | Quartz Flask | Ring of Invincibility | Shadowsphere | Timebomb of the Ancients | Tome of Power | Torch | Wings of Wrath