12.7x40mm

An M225 SAPHE from The Art of Halo 3.

The M225 Semi-Armor-Piercing High-Explosive (M225 SAP-HE) is a 12.7×40mm round used in the M6 series of sidearms.^[1]

Overview

The M225 SAP-HE is the standard issue ammunition for the M6 series of sidearms. Performance-wise, the round is effective against infantry, especially against unarmored or unshielded opponents. Compared to the M118 7.62×51mm and the M634 9.5×40mm rounds, the M225 is a relatively effective round against energy shields due to its high explosive ammunition properties. It is largely ineffective against vehicles, as it is not significantly armor-piercing.

Properties

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 ammo packs for the M6D in Halo: Combat Evolved have a type error: it says 'HP' rather than 'HE'.
The concept art in Art of Halo 3 refers the M225 SAP-HE as the 12.7×30mm instead of 12.7×40mm. The image of the round also reflects this error, as its short dimensions more closely correspond to the former than the latter. This type error carried on in the Magnum entry of Halo: The Essential Visual Guide.