The XM8 was the US Army project name for new lightweight assault rifle system. US Army hoped to create new standard and highly modifiable infantry rifle but in 2005, the project was formally canceled. ~7500 prototypes of this weapon still exist, now held by the US Army.
The weapon was designed to use 5.56x45mm NATO cartridges and is able to fire up to 750 rounds per minute.
As mentioned in its Armoury Description, the real-life "XM8" was originally slated to replace both the "M16" and "M4" series of weapons in use throughout the U.S. Armed Forces prior its formal cancellation in 2005.
Hidden models and textures for the XM8 were previously available in ArmA: Armed Assault. They were eventually removed after the release of Patch 1.08 however, which implies that they were intended to make their debut in that game instead of ArmA 2.
Along with the Mk16/Mk17 family, it is one of the few modular rifle platforms that are currently available for use in ArmA 2. It would only be matched by the MX in ArmA 3, which is almost identical to the XM8 series in terms of the types of variants that it has.
The ability to load and feed from Beta-C drum magazines is a capability that is unique to both the G36's MG36 variant as well as the SPAR-16S that would eventually be added with ArmA 3's Apex DLC.
Though it was actually included on ArmA 2's release, the XM8 family only included the basic variants without additional optical sights and accessories while also remaining unused by any faction prior to the release of the Private Military Company DLC.