Special Report

The US Military Rejected These Prototype Guns

Chris Lange

Published: December 26, 2023 9:25 am

When it comes to military and defense technology, the United States is on the cutting edge, with the development of new weapons systems being a consistent focus of U.S. Army designers. Whether it is the development of new aircraft or submarines, the U.S. is not only at the forefront with the newest advancements, but they are the country with the most state-of-the-art technology in the world. The Department of Defense (DOD), under the Office of the Under Secretary of Defense for Research and Engineering, and in conjunction with the Indiana National Guard even holds a Technology Readiness Experimentation (T-REX) to assess and accelerate technologies that the DOD deems vital. (Here is the evolution of automatic rifles in the U.S. Army.)

Not all weapons make it through, though, for a variety of reasons. 24/7 Wall St. reviewed Experimental Infantry Arms (X-Guns), cataloged by Military Factory, an online database of military vehicles, aircraft, and arms to identify the experimental small arms designed for the U.S. military that were never widely adopted. 46 prototypes did not pass muster; 25 of these were American-made. We have ranked 17 of these America-made weapons according to when the prototype was first tested. Supplemental data on the type of weapon, maximum effective range, and the caliber used by each gun also came from Military Factory.

The Barrett XM109 OSW, which expands on the Barrett M82/M107 anti-materiel rifle family, was one of these experimental designs, combining a sniper rifle with a grenade launcher and is chambered to fire 25x59mm smart grenades. While the body style and range of this weapon are akin to that of traditional sniper rifles, this weapon is designed to destroy whatever it hits. The smart grenade ammunition is also capable of featuring variable warhead loads for different combat scenarios. The XM109 was reportedly folded into a larger anti-materiel rifle program in 2006.

Throughout the 1990s, the U.S. Army sought to replace its long-standing M16 series through the Objective Individual Combat Weapon program. One of the prototypes was the Heckler & Koch HK XM29, which was a combination assault rifle featuring an under-barrel grenade launcher. Trials of this weapon were underway in 1995, but it would not make the cut and would be canceled for good in 2004. 

It is worth noting that these prototypes and experimental weapons on this list had mixed results in terms of implementation, sometimes inspiring a later, more successful model, and other times failing. However, each played a role in the development and evolution of small arms for the U.S. Armed Forces. One other noticeable feature of this list is that as time has gone on and technology has advanced, these weapons tend to gain longer ranges. (Also see, the guns in the U.S. Navy SEAL arsenal.)

The U.S. military rejected these prototype guns

17. Stoner 63 (Universal Combat Weapon)

Source: Public Domain / Wikimedia Commons

• Type: Modular assault weapon
• Maximum effective range: 1,800 ft.
• Caliber: 5.56x45mm NATO
• First year of prototype trials: 1963

16. Colt XM148 UBGL

Source: Public Domain / Wikimedia Commons

• Type: Under-barrel grenade launcher
• Maximum effective range: 800 ft.
• Caliber: 40x46mm
• First year of prototype trials: 1966

15. Aerojet XM174

Source: Public Domain / Wikimedia Commons

• Type: 40mm automatic grenade launcher
• Maximum effective range: 1,150 ft.
• Caliber: 40x46mm
• First year of prototype trials: 1968

14. China Lake Grenade Launcher

China Lake 4x40 REMOV (CC BY 3.0) by Remigiusz Wilk (REMOV)

• Type: 40mm pump-action grenade launcher
• Maximum effective range: 1,150 ft.
• Caliber: 40x46mm
• First year of prototype trials: 1968

13. AAI Inline Machine Gun (IMG)

Source: Public Domain / Wikimedia Commons

• Type: Multi-shot weapon system
• Maximum effective range: 3,280 ft.
• Caliber: 7.62x51mm NATO
• First year of prototype trials: 1970

12. Bushmaster Arm Pistol

Gwinn Arms Bushmaster pistol (CC BY-SA 3.0) by Amendola90

• Type: Bullpup personal defense weapon
• Maximum effective range: 325 ft.
• Caliber: 5.56x45mm NATO
• First year of prototype trials: 1977

11. Heckler & Koch HK CAWS

HK CAWS (CC BY-SA 3.0) by Mulhollant

• Type: Select-fire automatic combat shotgun
• Maximum effective range: 500 ft.
• Caliber: 12-gauge
• First year of prototype trials: 1983

10. AAI CAWS (Close Assault Weapon System)

Source: Jessica McGowan / Getty Images News via Getty Images

• Type: Assault shotgun
• Maximum effective range: 500 ft.
• Caliber: 12-gauge
• First year of prototype trials: 1985

9. Ares Defense FMG (Folding Machine Gun)

Source: Public Domain / Wikimedia Commons

• Type: Compact submachine gun
• Maximum effective range: 250 ft.
• Caliber: 9x19mm Parabellum
• First year of prototype trials: 1986

8. Pancor Jackhammer

Source: Public Domain / Wikimedia Commons

• Type: Automatic shotgun
• Maximum effective range: 500 ft.
• Caliber: 12-gauge
• First year of prototype trials: 1987

7. AAI ACR (Advanced Combat Rifle)

Source: Public Domain / Wikimedia Commons

• Type: Assault weapon
• Maximum effective range: 2,000 ft.
• Caliber: 5.56x45mm Flechette
• First year of prototype trials: 1989

6. Colt ACR (Advanced Combat Rifle)

Source: Public Domain / Wikimedia Commons

• Type: Assault weapon
• Maximum effective range: 1,800 ft.
• Caliber: 5.56x45mm NATO
• First year of prototype trials: 1989

5. Heckler & Koch HK XM29 OICW

Source: Public Domain / Wikimedia Commons

• Type: Combination assault rifle
• Maximum effective range: 3,280 ft.
• Caliber: 5.56x45mm / 20x85mm
• First year of prototype trials: 1995

4. General Dynamics XM307 (ACSW) Advanced Crew-Served Weapon

Source: Public Domain / Wikimedia Commons

• Type: Automatic grenade launcher
• Maximum effective range: 6,560 ft.
• Caliber: 25x39mm
• First year of prototype trials: 2004

3. General Dynamics XM312

Source: Public Domain / Wikimedia Commons

• Type: Heavy machine gun
• Maximum effective range: 6,560 ft.
• Caliber: .50 BMG
• First year of prototype trials: 2004

2. General Dynamics XM806 (LW50MG)

Source: Public Domain / Wikimedia Commons

• Type: Heavy machine gun
• Maximum effective range: 6,560 ft.
• Caliber: .50 BMG
• First year of prototype trials: 2009

1. Barrett XM109 OSW (Objective Sniper Weapon)

Source: Public Domain / Wikimedia Commons

• Type: Anti-material grenade launcher/rifle system
• Maximum effective range: 6,335 ft.
• Caliber: 25x59mm
• First year of prototype trials: 2014