Minigun bullets are a special type of ammunition used only for the XM-214-A Minigun. This is likely for balance reasons so that the player won't be swimming in minigun ammo because ammo for the M29 Infantry Assault Rifle is common.
In Serious Sam 3: BFE, minigun bullet boxes give 100 rounds of ammo (200 on Tourist, 150 on Easy) when picked up. Minigun bullet boxes are pretty uncommon throughout the game normally, but some can also be found in secrets, allowing the player to use the minigun more often if they go hunting for secrets.
In Serious Sam Advance, minigun bullets give 50 rounds per pickup. Minigun bullets are usually found in groups of two or so. They start appearing as soon as the player picks up the minigun and continue to be somewhat common throughout the game.
The below list shows how many of this item appears in each game.
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