Why Did The US Military Abandon The H&K XM8 & XM29?

From ACR conclusions to the OICW concept

The discussion begins with the US military’s long search for a replacement for the M16, following earlier efforts like the SPIW and ACR programs. By the mid‑1980s, a report on the ACR program concluded that conventional small arms technology had essentially peaked and that it would be difficult to field a new kinetic, ballistic weapon that clearly outperformed the M16. Planners began exploring the idea of issuing individual soldiers an explosive primary weapon instead of relying solely on bullets. This concept built on earlier ideas such as rifle grenades and the Close Assault Weapon System (CAWS). H&K’s CAWS prototype, essentially a bullpup shotgun, influenced thinking that would feed into the later Objective Individual Combat Weapon, or OICW, program that formally started around 1993.

Design of the XM29 combined weapon

When the OICW program launched in 1993, two main industry groups submitted designs: one led by AI, which included FN, and another led by Alliant Techsystems (ATK), which included Heckler & Koch. The Army favored the ATK/H&K team, and their design evolved into the XM29. The XM29 was conceived as a combined weapon system pairing an over‑barrel, magazine‑fed grenade launcher with an underslung kinetic energy weapon, essentially a compact rifle. This inverted the familiar M16 or M4 with M203 layout by making the grenade launcher the primary armament. The rifle portion was derived from the H&K G36, using a plastic chassis and a short barrel. The two components could be separated and used independently with their own stocks and fire control units, allowing a soldier to carry only the rifle or only the launcher depending on mission needs.

Airburst fire control and battlefield role

The XM29’s most ambitious feature was its integrated fire control system. A ballistic computer with a laser rangefinder sat atop the weapon, allowing the shooter to precisely measure distance to a target. The 20mm grenades could then be electronically programmed to detonate in different modes, including airburst. In airburst mode, the grenade was set to explode after traveling a specific distance, ideally three to six feet above or just beyond cover, producing a 360‑degree fragmentation effect. This was intended to defeat enemies behind walls, in trenches, or obscured by smoke or fog, where traditional impact‑fuzed grenades or rifle fire might be ineffective. The system also allowed impact detonation for direct fire against targets such as lightly armored vehicles. The sight could adjust the programmed burst point by a few meters plus or minus, letting the shooter compensate for targets moving or taking cover.

Weight, cost, and performance problems

Despite its advanced concept, the XM29 failed to meet key program requirements. The complete system weighed around 18 pounds at its lightest configuration, and practical field loads with optics, accessories, and ammunition pushed the weight even higher, far above an M16 with a typical combat load. Cost was also significantly greater than a standard rifle, although some analyses noted that once night vision, infrared devices, and other electronics were added to an M16 or M4, the price gap narrowed. Performance issues proved more serious. The 20mm grenade did not deliver the level of effect on targets that developers had hoped for. Meanwhile, the underslung 5.56mm rifle, firing M855 from a barrel only about 9.8 inches long, suffered from poor ballistic performance at distance. Soldiers were still expected to engage out to 200–300 yards with the rifle, but the short barrel limited effective range and terminal effect.

Splitting XM29 into rifle and launcher programs

Because the XM29 could not satisfy weight, cost, and performance goals as a single integrated system, the military decided to divide the effort into two related but separate development tracks. One track focused on creating a new rifle family derived from the kinetic energy portion of the XM29. The other concentrated on refining the grenade launcher concept as a stand‑alone weapon. The long‑term idea was to eventually return to a third phase in which a mature rifle and an improved launcher might be recombined into a new integrated system. In practice, that reintegration phase never occurred. The XM29 program was cancelled before the envisioned step three, and the rifle and launcher efforts continued on their own paths during overlapping phase one and phase two work.

XM8 modular rifle family and M320 launcher

The rifle component evolved into the H&K XM8, again drawing heavily from the G36 with a full polymer chassis. The XM8 was designed as a modular family that could be configured for multiple roles using a common core. Configurations included a very short‑barreled personal defense weapon, a carbine with a barrel around 12.5 inches for general issue, a designated marksman variant with an approximately 18‑inch barrel, and a light machine gun version with a 20‑inch barrel. The goal was for end users to reconfigure the platform to suit different roles within a unit while maintaining parts commonality. In parallel, a new grenade launcher compatible with the XM8 family was developed, which became the M320. The M320, similar in size and effect to the M203 but side‑loading instead of rear‑loading, ultimately entered service as a replacement for the M203, representing one concrete adoption from the broader OICW effort.

Alternate configurations and MP7 experiments

Before the XM29 design was standardized, developers experimented with alternative ways to pair the grenade launcher with a secondary firearm. One notable concept mounted the launcher over a weapon similar to the H&K MP7 instead of a G36‑based rifle. The MP7‑type platform used the 4.6mm cartridge, intended to defeat certain types of soft body armor, and functioned more like a compact personal defense weapon than a full‑size rifle. In this configuration, the grenade launcher remained the primary offensive tool, while the 4.6mm firearm served mainly as a close‑range personal defense option. This approach aligned with the idea that the explosive system, not the kinetic weapon, would carry most of the combat load. Although this specific combined configuration was never adopted, the MP7 itself went on to see independent service in various forces around the world.