From Halopedia, the Halo wiki
M6 series pistols, the weapons that fire this ammo, are noted to possess impressive range and stopping power due to its use. As with many UNSC small arms ammunition types, however, the round has been observed to be somewhat inefficient against energy shielding. There exists a distinct lack of variety in ammunition types of this cartridge available to soldiers in the field, something that the UNSC marines of E2-BAG/1/7 found strange given its high caliber.
M225 12.7×40mm SAP-HE (semi-armor-piercing, high-explosive) ammo (also known simply as “SAPHE”) is the standard-issue round for the M6-series. The rounds are nickel-plated, and are frequently distributed under the designation “M225 12.7×40mm SAP-HP” in 60-round packs.[note 2] It was noted to have made the weapon that fired it almost as effective as an assault rifle. Its physical effects against vulnerable targets can be brutal, particularly when the projectile impacts the target's skull, making headshot-targeting with weapons firing this ammo an advisable tactic in combat with Covenant troops, who often possess resilient biology. This ammo type is fed into M6 series handguns via 6-, 8-, or 12-round detachable box magazines.
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