The M228 Semi-Armor Piercing high-Penetration is a round used in the M6 series sidearms.
It is somewhat rare and more of an exotic round due to its rarity of use compared to the M225 SAPHE. [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 exiting the body and causing accidental casualties. The filler is likely a hard or heavy metal, such as tungsten, which would provide the extra force and penetration this round boasts of.