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.

In terms of muzzle energy, the round may be as powerful (or even more powerful) than even the mighty .50 caliber pistol rounds of today, possibly giving a muzzle energy of 3,000 foot pounds of force or more (though possibly less as well). While it may seem like the M6 pistols would be totally overpowered by today's standards, strides in recoil reduction, accuracy, etc., may very well have been made in the five-hundred year gap in firearms technology of today to Halo technology. Also, advances in defensive measures such as stronger body armor might mean handguns of lesser power have become inferior, at least for military combat purposes.

Current .50 caliber bullets have severe recoil and although MJOLNIR armor may increase strength to a point where it is manageable, UNSC servicemen are able to use M6 pistols easily. Contemporary .50 handguns are not militarily viable, however advancements in military technology allows this caliber of round to perform very well in the M6 line of handgun.

Two different versions of this cartridge exist, one using a 40mm case and another using a shorter 30mm case. The longer case of the 12.7x40mm cartridge provides the ability to provide greater muzzle energy and velocity as it has a larger case volume allowing more propellant to be stored inside. The shorter 30mm case variant likely produces a less powerful shot, with the benefit of decreased recoil, allowing for faster follow up shots. Both rounds are used in the M6 series of pistols and possibly other weapons.

Trivia

 * The 12.7mm ammo is similar to the .50 Action Express round most famously used in the Desert Eagle, however the bullets are not the same. The .50 Action Express is a 12.7mm x 33mm. It is not to be confused with the the .50 BMG, which is a 12.7mm x 99mm rifle round, used by heavy machine guns such as the M41 Light Anti-Aircraft Gun.
 * The round could possibly be a development of the .500 S&W Magnum cartridge, since they are the same caliber and length, although the cartridge is rimmed and it is a revolver cartridge, making it unsuitable in a semi-automatic pistol like the M6.
 * The cartridge is more than likely based on the .50 Beowulf, since it has a rebated rim and it is the same caliber and length as the .500 S&W Magnum. However, .50 Beowulf was designed for use in a modified AR-15 type rifle, and like the .500 S&W Magnum, is too powerful to be used in a light sidearm.
 * There is no such thing as a "Semi Armor-Piercing" bullet in modern firearms; rounds are either AP or not. Semi Armor-Piercing may mean that the round is capable of penetrating only light body armor.
 * Since the game specifies that the M6D, the pistol that fires this cartridge, is 26.7cm long, the actual rounds it should handle are actually 9 or 10mm, to actually being capable of firing 12.7 rounds, the pistol must be 33.39cm long making it very bulky, just about the size of a machine pistol or submachinegun, and is probably only suitable for Spartans and not marines.
 * In Halo, if you look closely at the ammo packs for the Pistol, you'll see that there is a typo on the bitmap, it says 'HP' rather than 'HE'.