The XM8 is a weapon featured only in Thing-Thing 4.
The XM8, developed by Heckler & Koch USA (the U.S. branch of the renouned German firearms company), originally started out as the underslung assault rifle component of the prototype XM29 Objective Individual Combat Weapon (OICW). When the XM29 didn't meet the weight, cost, and performance requirements, it was split into two separate weapons, the XM25 Counter Defilade Target Engagement (CDTE) System, and the XM8 Lightweight Assault Rifle.
The XM8 was designed to be a standard next-generation assault rifle to replace the aging M16 and M4 families, that is lightweight, cost-effective, and easy to maintain. Sadly, the project was cancelled on October 31st, 2005, but the weapon is seeing service with Malaysian special forces units and some private military companies.
The XM8 fires the NATO-standard 5.56x45mm cartridge, and weighs 7.5lbs, 20% lighter than the M4A1 that it had failed to replace.
Performance-wise, the XM8 isn't as powerful as weapons using larger rounds such as the 7.62x39mm or the .308 Winchester, but it has a high rate of fire and is very accurate, along with having good reach. It is very much identical in performance to another weapon from the same company, the Heckler & Koch HK416.
- The XM8 featured in the game did not come with the telescopic buttstock as it did in real life, but considering the way that the guns are designed throughout the series, it wouldn't be surprising.
- While the XM8 is an assault rifle, the XM25 CDTE is a grenade launcher firing 25mm airburst rounds (larger than the 20mm rounds fired by the XM29's grenade launcher component from which the weapon was derived).