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M634 Experimental High-Powered Semi-Armor-Piercing (abbreviated M634 X-HP-SAP) ammunition is a type of United Nations Space Command rifle projectile that is made specifically for the UNSC BR55 Battle Rifle series. The round's metric dimensions are 9.5x40mm, compared to the 7.62x51mm NATO rounds used in the MA5 Series of UNSC Assault rifles, and was introduced into the UNSC Marine Corps along with the BR55 Service Rifle.
In combination with the BR55 Battle Rifle, this round is accurate enough that the 2x scope on the Battle Rifle is useful. Even when fired in three-shot bursts, the round produces little recoil resulting in very little muzzle lift; it is more effective against both shielded and unshielded Covenant personnel than the 7.62x51mm NATO, as the increased accuracy leads to an increased number of headshots and the increased kinetic energy of the round leads to faster shield depletion and increased damage to unshielded enemies. It has been shown that a BR55 Service Rifle with M634 ammunition does enough damage to kill an opponent faster than an MA5C Assault rifle using 7.62x51mm NATO ammunition. Overall, the M634 Experimental High-Powered Semi-Armor-Piercing round provides good accuracy, controllability, and effectiveness against all personnel.
However, while the M634 X-HP-SAP does do more damage than the 7.62x51mm NATO against vehicles, it is still not a very effective round at damaging or stopping vehicles.
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 7.62x51mm NATO 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 Semi-Armor-Piercing High-Explosive used in the M6 Series.
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