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 personnel are able to use M6 pistols easily. .50 caliber handgun cartridges are not efficient or useful in combat and are not used by current military.

Trivia

 * The 12.7mm ammo is used in modern weapons, mostly by sniper rifles and a high caliber Desert Eagle handgun, 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. Sniper rifles and machine guns use the .50 BMG, which is a 12.7mm x 99mm rifle round, even more powerful.