M634 Experimental High-Powered Semi-Armor-Piercing
M634 Experimental High-Powered Semi-Armor-Piercing, abbreviated as M634 X-HP-SAP or M634 9.5×40mm HP-SAP, and also known as the 9.5×40mm KURZ,[note 1] is a 9.5×40mm rifle cartridge made specifically for the BR55 series and BR85 series.
The M634 X-HP-SAP was developed in conjunction with the prototype XBR55 Battle Rifle. As an experimental round, its use is only limited to the BR55 Battle Rifle, the BR55 Heavy Barrel Service Rifle, and the BR85 Heavy Barrel Service Rifle. While the round was designated as experimental around 2525, at some point between then and 2558, the "experimental" designation was dropped, becoming the M634 9.5×40mm HP-SAP.
As designated in its name, the M634 X-HP-SAP is an experimental round. It uses a special propellant that allows it to achieve a higher muzzle velocity than the M118 7.62×51mm round while using a smaller cartridge. It is also semi-armor-piercing, meaning that it mixes the properties of an armor piercing round with the properties of another type of round. The second type is unknown, though it is most likely similar to the M225 SAP-HE used in the M6 Series.
Unlike most ammunition types in the Halo games, this is not a real-life ammo type, the closest comparison being the 9×39mm special purpose round. Designed in the Soviet Union, this round was designed to defeat body armor at ranges closer than 100 meters, and be subsonic, so it can be easily used in suppressed firearms; it appears most real-life weapons firing the bullet have integrated silencers. However, the M634 does not appear to be subsonic, instead intended for high-velocity and long-range marksmanship. The M634 is likely the result of human inclinations towards increased size (leading to the 9.5 mm bullet) and technological manipulation (leading to the experimental propellant). It's also possible the large caliber, while overkill by current standards, is required to defeat the advanced body armor of the Insurrectionists, and later, the Covenant. Additionally, the round is very similar in dimensions to the .375 SOCOM round, which came out after the battle rifle first premiered in Halo 2, but shares its 9.5x40mm cartridge measurements. At some point before 2558, the experimental identification of the M634 was removed, suggesting the round received formal adoption by the United Nations Space Command.
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