The XM8 is a developmental U.S. military designation and project name for a lightweight assault rifle system that was under development by the United States Army from the late 1990s to early 2000s. The U.S. Army worked with the German small arms manufacturer Heckler & Koch (H&K) to develop the system to its requirements in the aftermath of the OICW contract, for which H&K had been a subcontractor to ATK. The XM8 is based on the H&K G36, using most of same parts.

Although there were high hopes that the XM8 would become the Army's new standard infantry rifle, the project was put on hold in April 2005, and was formally canceled on October 31, 2005.

Cartridge 5.56x45mm NATO
Action Gas-operated, rotating bolt
Rate of fire 750 rounds/min
Muzzle velocity 920 m/s
Feed system 30-round detachable box magazine, 100-round C-Mag drum magazine
Sights Unmagnified reflex sight (4x for DMR variant)