From Halopedia, the Halo wiki
The Sniper, formally designated Supplementary bandoleer, Semi-rigid, Special Applications Rifle (Anti-materiel), is a shoulder armor attachment designed for carrying sniper rifle ammunition. The accessory is compatible with the MJOLNIR Mark V PAA.
Each shoulder bandoleer holds three sniper rounds, presumably to facilitate the reloading of half-empty magazines for the rifle. The ammunition is High Velocity Armour Piercing Rounds, measuring 144mm in length, and 14.5mm in diameter, for SRS99-AM sniper rifles.
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