The 12.7x40mm M228 Semi-Armor-Piercing High-Penetration (abbreviated M228 SAP-HP) is a type of .50-caliber round used in the M6 Series or sidearms.
It is somewhat rare and more of an exotic round due to its rarity of use compared to the M225 SAP-HE.[1]
Due to the semi-armor-piercing nature of the round, it is likely coated in a hard metal such as steel but deforms shortly after impact to prevent the bullet from leaving the body and causing accidental casualties by overpenetrating a target. The filler is likely a hard or heavy metal, such as tungsten or nickel, which would provide the extra force and penetration this round boasts of.
In Halo multiplayer, the M228 SAP-HP is drastically underpowered, taking almost 13 rounds (which is one round larger than the magazine of a pistol) to kill a fully shielded Spartan or Elite.