From Halopedia, the Halo wiki
11.43mm (.450) is a non-standard round occasionally used in the Sniper Rifle System 99C-S2 AM. It is considerably smaller and lighter than the standard 14.5×114mm round, and ostensibly still capable of dealing considerable damage to a target. This round is not the SRS-99C S2 AM rifle's default chambering, as the UNSC's standard cartridge for the weapon is 14.5×114mm.
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