M228 Semi-Armor-Piercing High-Penetration

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 sidearms.

It is somewhat rare and more of an exotic round due to its rarity of use compared to the M225 SAP-HE.

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.

Trivia

 * The 12.7 is a real-life round type, used mainly in Sniper Rifles.