From Halopedia, the Halo wiki
The M645, contained in a rectangular casing, is loaded into the railgun's internal magazine through a topside well. The slug contains a high-explosive charge to aid in armor penetration. Coupled with the kinetic force imparted by the weapon's high muzzle velocity, the M645 can penetrate both hard and soft targets; ideally it is to be used against well-armored personnel. The M645 FTP-HE has helped dramatically decrease the mass of the ARC-920. The follow-on explosive charge in the M645 has helped make the ARC-920 railgun the most popular electromagnetic launcher among augmented UNSC Armed Forces personnel.
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