12.7x40mm

The 12.7x40mm M225 Semi-Armor-Piercing High-Explosive (abbreviated M225 SAP-HE) is a type of .50-caliber round used in the M6 series of sidearms.

Due to the semi armor piercing nature of the round it is likely coated or  made of 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 stable explosive with either a contact explosive detonation (somewhat counter acting its SAP nature) or a delay action fuse so it goes off shortly after impact, ideally in the body of the enemy, often lethal and crippling.

Current .50 caliber bullets have severe recoil and although MJOLNIR armor may increase strength to a point where it is manageable, UNSC servicemen are able to use M6 pistols easily. Actual .50 caliber handgun cartridges are inefficient, inaccurate, uncomfortable to use and are unlikely to be used by any military.

Trivia

 * The 12.7mm ammo is similar to the .50 Action Express round most famously used in the Desert Eagle, however the bullets are not the same. The .50 Action Express is a 12.7mm x 33mm, making the M6 series pistols significantly more powerful. It is not to be confused with the the .50 BMG, which is a 12.7mm x 99mm rifle round, used by heavy machine guns such as the M41 Light Anti-Aircraft Gun.
 * The round could possibly be a development of the .500 S&W Magnum cartridge, since they are the same caliber and length, although the cartridge is rimmed and it is a revolver cartridge, making it unsuitable in a semi-automatic pistol like the M6.
 * The cartridge is more than likely based on the .50 Beowulf, since it has a rebated rim and it is the same caliber and length as the .500 S&W Magnum. However, .50 Beowulf was designed for use in a modified AR-15 type rifle, and like the .500 S&W Magnum, has extreme recoil.
 * There is no such thing as a "Semi Armor-Piercing" bullet in modern firearms; rounds are either AP or not.