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.[1]

It was used by Linda-058 when Blue Team captured Colonel Robert Watts. Linda apparently converted her rifle with an interchangeable barrel assembly in order to use this cartridge.[2]

List of appearances[edit]