M634 Experimental High-Powered Semi-Armor-Piercing

M634 Experimental High-Powered Semi-Armor-Piercing (abbreviated M634 X-HP-SAP) ammunition is a type of United Nations Space Command .374-caliber rifle projectile that is made specifically for the UNSC BR55 Battle Rifle series.

Introduction
Introduced along with the BR55 Battle Rifle as an improvement over the standard 7.62x51mm assault rifles in the UNSC's inventory, this intermediate round provides good accuracy, controllability, and effectiveness against most Covenant personnel. Unlike most ammunition types in the Halo games, this is not a real-life ammo type.

Physics
The 9.5mm cartridge's performance is consistent with other special-purpose rifle calibers, offering high power and accuracy in close-combat engagements. In a rifle weighing anywhere between 8.5 to 11 pounds, this would produce manageable felt recoil that could be placed somewhere between that of the modern-day AK-47 and a full-power rifle such as the FN FAL, with a muzzle energy of around 3,200 joules, although it is only an estimation based on the width of the cartridge and how many rounds it takes to kill an opponent.

The 9.5x40mm cartridge is used in standard 36-round magazines for the BR55, providing the shooter with twelve bursts before a reload is required. Though fire selection is disabled in game, for simplicity of gameplay and balance, in the Halo Shorts, the BR55 is capable of fully automatic fire.

The only weapon that uses this caliber cartridge is the BR55 Battle Rifle, which first appeared in Halo 2 and returned in Halo 3. The fact that the Battle Rifle does much more damage than the original Assault Rifle could have to do with the higher knockdown power and kinetic energy transfer effect of the larger projectiles.

Overall, the 9.5mm x 40 Experimental Armor-Piercing round is an excellent medium-range intermediate cartridge, providing comparable effectiveness to the antiquated Russian 9x39mm round. The 9x39mm SP-5 and SP-6 subsonic armor piercing cartridges, which were developed for a series of special purpose sniper rifles and compact assault rifles, are also similar in use to the 9.5x40mm cartridge in the BR55. In many ways, the Battle Rifle is more of a true Assault Rifle than the MA5 Series, as its shorter, more controllable cartridge provides it with a great deal of accuracy and effectiveness against armored and energy-shielded foes in a wide variety of situations. However, rifle cartridges of this caliber are likely much more powerful than more usual calibers like 7.62mm or 5.56mm. For it to be more 'controllable' than previous rifle rounds, either the BR55 or the cartridge must incorporate some highly effective recoil-damping system that is not discussed or externally visible, as the weapon lacks a noticible muzzle brake. The three prongs on the barrel would be a flash hider rather than for recoil control.

The 9.5mm x 40 Experimental Armor-Piercing round has a shorter cartridge and a larger round than the 7.62mm NATO which should make the round bulkier, decrease muzzle velocity, and shorten the range. However the 40mm case has been downsized to be more compact and could have a more efficient powder hence solving the issue with the smaller case.