12.7x40mm
From Halopedia, the Halo wiki
Template:Ratings 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. [1]
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.
Trivia
The 12.7mm ammo is used in modern weapons, usually sniper rifles.
