M634 Experimental High-Powered Semi-Armor-Piercing

9.5x40mm 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.

Physics
The 9.5mm cartridge's performance is consistent with other special-purpose rifle calibers, offering high power and accuracy in close-combat engagements. The maximum effective range of the BR55 Battle Rifle against energy-shielded targets is around 200 meters. This is fairly limited compared to most modern 7.62x51mm Main Battle Rifles, which leads us to believe that the 9.5mm cartridge is a special medium-velocity, medium-mass round of approximately 200-220 grains/13.0-14.2 grams weight, traveling at a velocity of around 1,800 to 2,200 fps/550 to 670 m/s. 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 2,500 joules, although it is only an estimation based on the width of the cartridge and how many rounds it takes to kill an opponent. For reference, one 14.5x114mm round from the SRS99C-S2 AM is equal to around 30,000 joules of muzzle energy, and since 2,500 joules times twelve rounds equals 30,000, it follows that each round from the BR55 possesses more energy than a 7.62x39mm round, but less energy than a real-world 7.62x51mm round.

Ironically, this makes for a bit of an inconsistency regarding damage ratios of the powerful 7.62x51mm round versus the 9.5mm. In Halo: Combat Evolved, it takes 16 direct headshots at close range to kill an opponent with the MA5B. Compare this with 12 rounds (four bursts) from the BR55. If the UNSC's weapons were to conform to real-life ballistics, each individual 7.62x51mm round from the MA5B Assault Rifle would actually do over 37% more damage and remain effective at much longer ranges than each 9.5x40mm round, at the expense of very poor controllability on fully-automatic fire.

The 7.62x51mm round should technically be capable of bringing down an energy-shielded opponent with almost half as many shots as the MA5B. 3,400-4,000 joules per round is equal to between 9 and 7 shots, respectively, per energy-shielded opponent. However, the effect of standard FMJ projectiles on energy-shielded targets may be poorly understood. It is entirely possible that the armor-piercing capacity and high velocity of the 7.62x51mm AP round cause it to over-penetrate the target's armor and glance off its shields without fully transferring its energy into the target, greatly limiting its wounding potential. Nevertheless, the Battle Rifle's most salient advantage over the MA5-series of weapons is its ability to place more armor-piercing rounds on-target, delivering a three-round burst in very small, accurate groups. To achieve the same accuracy in a MA5x-series weapon, you must fire in either single shots, or two-round bursts, both of which possessing less total kinetic energy delivered on-target than with the BR-55's distinctive three-shot burst.

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. Why UNSC weapon designers chose to omit a fire selector for the weapon is unknown, but the fact remains that the rifle would surely benefit from the ability to fire in single-shot and full-auto modes, despite the limited effectiveness of single shots against energy-shielded targets.

The only weapon that uses this caliber cartridge is the BR55 Battle Rifle, which first appeared in Halo 2 and will be returning 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 ancient Russia 9x39mm. The 9x39mm SP-5 and SP-6 subsonic armor piercing cartridges, which were delevoped 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 MA5x-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.