The 5.56×57mm XM645 was an experimental military cartridge developed by AAI for Project NIBLICK.
The XM645 is a redesign of the older XM110 round. The casing is 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.