The XM29 OICW was a series of prototype of assault rifle that fired 20mm HE airbursting projectiles. The prototypes were developed as part of the Objective Individual Combat Weapon program in the 1990s. The term SABR (Selectable Assault Battle Rifle) was also used at certain points, but is less common.
Developed by Alliant Techsystems, with Heckler & Koch as a major subcontractor, the most commonly seen version of the XM29 consisted of a semi-automatic 20x28mm smart grenade launcher, and underslung assault carbine (derived from the HK G36 then in its late developmental stage) firing a standard 5.56x45mm NATO round, and a top-mounted computer assisted sighting system with integrated laser rangefinder, thermal vision/night vision capabilities, and up to 6x optical telescopic sight. Earlier designs used different configurations and setups.
The launcher part has been described variously as a light semi-automatic 20mm cannon, a grenade launcher, or an airburst weapon. It poses a classification problem, in that it does not fit neatly into any one category. On one hand, it uses much smaller shells and has a much flatter trajectory than grenade launchers. On the other, while its caliber and velocity is much more similar to light cannon, it does share trais with other infantry grenade launchers.
Various problems have effectively ended the program as it was originally envisioned, including weight, bulk, and lack of effectiveness of the 20x28mm airburst round. Upon cancellation in 2004, it was split into three related programs. OICW Increment One is a program to develop a family of light kinetic energy weapons. OICW Increment Two is a program to develop the airburst grenade launcher as a standalone component, and OICW Increment Three will be a program to re-integrate the two components. The XM8 was developed in an attempt to meet Increment One requirements.