The Heckler & Koch HK XM8 was an experimental modular assault rifle system that underwent trials with the US Army in an effort to become its next standard-issue assault weapon system. The firearm was to replace the long-running Colt M16 rifle and M4 carbine forms. As such, the XM8 was to become its own family of automatic weapons to include the basic assault rifle (that could also double as a designated marksman rifle), a shortened carbine (with support for the experimental XM320 grenade launcher), a dedicated sharpshooter's version and a compact carbine with optional collapsing stock. Despite the program's promising results, the XM8 was suspended in April of 2005 before seeing official cancellation in October of that year.
The XM8 family would have comprised of a 9-inch compact, 12.5-inch carbine and a 20-inch sharpshooter and automatic rifle forms. The 12.5-inch carbine was 6.4 pounds with an objective of 5.7 pounds and fielded a running length of 33 inches with its adjustable stock fully extended. In comparison, the M16A2 was 39.63 inches long with a 8.79lb weight and full 30-round magazine in place. Internally, the XM8 utilized a gas-operated, rotary locking bolt action hat functioned and fieldstripped similarly to the M16 rifle and M4 carbine to was meant to replace. The bolt was powered by a unique gas operating system with a user-removable gas piston and pusher rod used to operate the mechanism (the system derived from the HK G36). Unlike the current M4 and M16 direct gas system with gas tube, the XM8 gas system did not introduce propellant gases and carbon back into the weapon's receiver during firing which would have cut down on fouling. The XM8 was chambered for the 5.56x45mm NATO standard cartridge and fed from a 30-round curved detachable box magazine. There was further support for the 100-round Beta C-Mag drum magazine as well.
Design work on the XM8 stemmed from 2002 to which prototypes were completed throughout 2003 and 2004. The XM8 returned to life once more for evaluations concerning the SCAR competition to which the XM8 lost out.