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 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 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.
