From Halopedia, the Halo wiki
MUTA-AP-09334 is a type of ammunition manufactured by humans and used in handheld machine guns. It is designated as armor piercing ammunition, and is only used when taking on light tanks or against enemies in advanced armor systems such as the MJOLNIR armor.
MUTA-AP-09334 is .30 caliber, allowing it to be used in "Confetti Maker" MGs. It is unclear if the UNSC uses this type of ammunition, but it has fallen into use by the Insurrectionists. During Operation: SMOKING ACES in 2531, Kurt-051 discovered General Howard Graves' elaborate ambush intended for Blue Team after noticing multiple crates labeled as containing MUTA-AP-09334 ammunition, inferring that the rebels would not normally need so much armor-piercing rounds unless they were prepared to face several light tanks or a team of Spartans.
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