The 5.56×57mm XM645 was an experimental military cartridge developed by AAI for the SPIW project.

Details[edit | edit source]

The XM645 is a redesign of the older XM110 round. The casing is now 4mm longer in length, while eliminating the concave bottom area of the casing structure. Early examples also implement a new single-piece piston primer, while later rounds used traditional Berdan primer caps.

Like the XM110 before it, the projectile is a steel flechette encased in a green fiberglass. Later examples of the ammunition has the flechette encased in a white synthetic bale.

The casing is also used as the basis for the 5.77×52mm SBR cartridge developed for the AAI SBR weapon.[1]

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