12.7x40mm
From Halopedia, the Halo wiki
The M225 Semi-Armor Piercing High Explosive is a 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.
