XM8 Lightweight Carbine
5.56mm Modular Assault Weapon System

The XM8 Lightweight Carbine was intended to replace existing M4 carbine and select 5.56x45mm and 9mm weapons in the US Army arsenal. The system during its development was alternately known as the Future Combat Rifle, Lightweight Assault Rifle, and expanded into a family of weapons known as Modular Assault Weapon System or Modular Assault Weapons Family, designated as the Objective Individual Combat Weapon's (OICW) Increment 1. The formal XM8 nomenclature, however, was Carbine, 5.56mm: Lightweight, XM8, and referred only to that configuration. The weapon was derived from the so-called "Kinetic Energy" component of the OICW.

The XM8, originally intended just as a lightweight carbine, was eventually expanded into a true family of weapons with different barrel lengths designed to address all the needs of an infantry squad. The standard model, a carbine with a 12.5 inch barrel, was expected to be lighter than the M4 carbine and no larger in size. There was also a special compact version with a 9 inch barrel for cramped quarters, along with a sharpshooter version for increased range, and an auto-rifle version for a squad-automatic weapon. The latter two weapons had 20 inch barrels. The carbine weighed 6.4 pounds with an objective of 5.7 pounds and was 33 inches long with its adjustable stock extended. The M16A2 was 39.63 inches long and 8.79 pounds with a loaded 30-round magazine.

Internally, the XM8 uses a rotary locking bolt system that functions and fieldstrips like those used in the M16 rifle and M4 carbine. The bolt is powered by a unique gas operating system with a user-removable gas piston and pusher rod to operate the mechanism. Unlike the existing 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.

The XM8 is a derivative of the Heckler and Koch G36 assault rifle, and is similar to that rifle in design and functioning. Heckler and Koch's XM8 entrant was based on the kinetic energy weapon that was part of the XM29 next-generation infantry weapon system (formerly the Objective Individual Combat Weapon), under development by a partnership between Alliant Techsystems Integrated Defense and Heckler and Koch. The kinetic energy weapon, which fires 5.56mm ammunition, would provide maximum commonality in components and logistics with the XM29 system.

Developed by the US Army's office of Project Manager for Soldier Weapons located at Picatinny Arsenal, New Jersey in close conjunction with the US Army Infantry Center, the XM8 Future Combat Rifle was intended to replace existing M4 Carbines and select 5.56x45mm weapons in the US Army arsenal. Once adopted, the M8 Carbine would replace the aging M16/M4 family of weapons, which had been in service for nearly 4 decades by 2000, longer than any previous US service rifle. The XM8 was developed to provide lethality performance to comparably equipped M4 carbine, while weighing 20 percent less than the M4 because of advanced technologies developed for the XM29 program. The prototype weighed 6.4 pounds, with an objective weight of 5.7 pounds. The Program Executive Office (PEO) Soldier claimed it took one third the time to train a soldier on the XM8 than the existing weapon system.

The XM8 was also more reliable. Unlike the existing M4 and M16, the XM8 did not introduce propellant gases and the associated carbon fouling back into the weapon's receiver during firing. This greatly increased the reliability of the XM8, and it reduced cleaning time by as much as 70 percent. PEO Soldier said the XM8 could fire 20,000 rounds without stoppages or malfunction. The first XM8 tested fired 15,000 rounds without cleaning or lubrication without a first misfire.

This improved reliability could be credited to differences in the XM8's operating system from the one in the M16. For instance, a thin gas tube runs almost the entire length of the barrel in all of the M16 variants. When the weapon is fired, the gases travel back down the tube into the chamber and push the bolt back to eject the shell casing and chamber a new round. The XM8's gas system instead is connected to a mechanical operating rod, which pushes back the bolt to eject the casing and chamber the new round each time the weapon is fired. As a result there is no carbon residue constantly being blown back into the chamber, reducing the time needed to clean the weapon up to 70 percent and an overall reduction in how often the weapon had to be cleaned. With the piston system one would not get gases blowing back into the chamber that have contaminates in them. The XM8 also has a much tighter seal between the bolt and the ejection port, which would cut down on the amount of debris that can blow into the weapon when the ejection port's dust cover is open.

As a direct development of the separable OICW's KE component, the XM8 Carbine was to share a high degree of common parts and training and maintenance procedures to lessen the required support for the "family" of XM8 weapons. Being developed were 4 XM8 variants, which included a baseline carbine, a sharpshooter variant, an automatic rifle variant, and the ultra-compact carbine variant. A unique feature of the XM8 modular weapon system was the ability to easily and quickly reconfigure the weapon from one variant to the other to meet changing mission requirements, to include caliber conversion.

This modularity includes the exchange of interchangeable assembly groups such as the barrel, handguard, lower receiver, buttstock modules and sighting system with removable carrying handle. In addition and in parallel, the new XM320 quick detachable single-shot 40mm grenade launcher with side-opening breech and XM26 Modular Lightweight Shotgun System (MASS) lightweight 12-gauge shotgun module could be easily added to the XM8 by the user in the field without tools. The unique buttstock system allowed the operator exchange buttstocks without tools from the standard collapsible multi-position version, to an optional buttcap for maximum portability or an optional folding or sniper buttstock with adjustable cheekpiece for special applications. A cold hammer forged barrel would guarantee a minimum of 20,000 rounds service life and ultimate operator safety in the event of an obstructed bore occurrence.

The XM8 has fully ambidextrous operating controls to include a centrally located charging handle that doubles as an ambidextrous forward assist when required, ambidextrous magazine release, bolt catch, safety/selector lever with semi and full automatic modes of fire and release lever for the multiple position collapsible buttstock. The operating controls allow the operator to keep the firing hand on the pistol grip and the weapon in the firing position at all times while the non-firing hand actuates the charging handle and magazine during loading and clearing. Major components of the weapon are produced from high-strength fiber reinforced polymer materials that can be molded in almost any color to include olive drab (OD green), desert tan, arctic white, urban blue, brown, and basic black. Surfaces on the XM8 that interface with the operator are fitted with non-slip materials to increase comfort and operator retention. The XM8 uses 10 or 30-round semi-transparent box magazines and high-reliability 100-round drum magazines for sustained fire applications.

Special integral flush mounted attachment points are located on the handguard and receiver to allow the quick attachment of targeting devices. Unlike MIL-STD-1913 rails, the XM8 attachment points do not add additional weight, bulk and cost to the host weapon, and will accept MIL-STD-1913 adapters to allow for the use of current in-service accessories. The attachment points for the standard multi-function integrated red-dot sight allow multiple mounting positions and insure 100 percent zero retention even after the sight is removed and remounted. The battery powered XM8 sight includes the latest technology in a red dot close combat optic, IR laser aimer and laser illuminator with back-up etched reticle with capability exceeding that of the existing M68 Close Combat Optic (CCO), AN/PEQ-2 and AN/PAQ-4. This sight would be factory zeroed on the weapon when it was delivered and would not require constant rezeroing in the field like existing rail-mounted targeting devices. The XM8 was also expected to be fully compatible with Land Warrior technology and components.

The XM8 Lightweight Carbine was intended to reduce the 21st century soldier's load and increase his mobility, 2 very important aims of the Army's Objective Force Warrior and Land Warrior initiatives. The progress made to reduce weight and improve performance on the XM29 program was key to the decision on accelerating the development of the XM8, which was integrated with the Army's efforts to transform to a more lethal and rapidly deployed fighting force as part of its Objective Force.

The XM8 carbine was designed at the Heckler and Koch Defense design center in Sterling, Virginia and was to be produced and assembled in the United States at the new Heckler and Koch manufacturing plant located in Columbus, Georgia, adjacent to Fort Benning. The unit cost of the XM8 would be less than that of the existing M4 Carbine and would guarantee the American war fighter uncompromising performance far exceeding that of current in-service M4 Carbines.

ATK Integrated Defense was the system integrator on the XM8 program. Teammates on the program were Heckler and Koch, assisting in weapon development; ATK Ammunition Systems of Arden Hills, Minnesota, assisting in ammunition development; Brashear LP of Pittsburgh, Pennsylvania, providing the integrated full solution fire control system; and Omega of Columbus, Georgia, providing the training systems.