Heckler & Koch XM - 8 The XM8 was an expirimental assault rifle which was supposed to replace the M16 . while the gun preformed remarkably , the program was canceled after complaints from HK about the non - competitive nature of the program . Specifications . The development of the XM8 Lightweight Assault Rifle was initiated by US Army in the 2002 , when contract was issued to the Alliant Techsystems Co of USA to study possibilities of development of kinetic energy part of the XM29 OICW weapon into separate lightweight assault rifle , which could , in the case of success , replace the aging M16A2 rifles and M4A1 carbines in US military service . XM8 is being developed by the Heckler - Koch USA , a subsidiary of famous German Heckler - Koch company . According to the present plans , the XM8 should enter full production circa 2005 , if not earlier , several years before the XM29 OICW . The XM8 ( M8 after its official adoption ) should become a standard next generation US forces assault rifle . It will fire all standard 5 . 56mm NATO ammunition , and , to further decrease the load on the future infantrymen , a new type of 5 . 56mm ammunition is now being developed . This new ammunition will have composite cases , with brass bases and polymer walls , which will reduce weight of the complete ammunition , while maintaining compatibility with all 5 . 56mm NATO weapons . Along with 20 % weight reduction in the XM8 ( compared to the current issue M4A1 carbine ) , this will be a welcome move for any infantryman , already overloaded by protective , communications and other battle equipment . The XM8 will be almost similar to the " KE " ( kinetic energy ) part of the XM29 OICW system , being different mostly in having a telescoped plastic buttstock of adjustable length , and a detachable carrying handle with the sight rail . As of mid - 2004 , XM8 now is undergoing wide - scale testing in several units of US Army , and is receiving a positive feedback . Following several complains about non - competitive procurement of the XM8 system from Heckler - Koch USA , US Army first opened the bidding for contract to other companies , and then , in the July 2005 , suddenly suspended the entire OICW Increment 1 program to re - think the entire set of requirements for the possible replacement of current M16 rifles and M4 carbines . These requirements will be re - viewed as Joint Services effort . At the present time , it seems that development of HK XM8 is suspended ; it may be resumed if XM8 suit the new US Joint Service Requirements for a new assault rifle system ; or , it may not . We will watch this process closely and update this article when new information will be available . Techinical Description . The XM8 is a derivative of the Heckler - Koch G36 assault rifle , and thus it is almost similar to that rifle in design and functioning . XM8 features a short piston stroke , gas operated action , with rotating bolt locking . Barrels are quick detachable , and planned to be available in several sizes , ranging from 229 mm ( 9 . 5 inch ) for Compact / PDW version , 318mm ( 12 . 5 inch ) in Basic version , and two 508mm ( 20 in ) barrels , one for Sharpshooter / Sniper version , and heavier one ( along with bipod ) for Squad Automatic Rifle role . The entire construction is modular and built around the polymer receiver with bolt group ; Magazine housings could be easily swapped for compatibility with various types of magazines ; various buttstocks could be installed in a second for various roles ( standard buttstock is a telescoped 5 position adjustable one ) . Top of the receiver is fitted with proprietary sight rail , which can accept illuminated red - dot ( collimator ) sight , or any other type of sighting equipment . Detachable forend will be available in various sizes , and could be replaced with XM320 40mm grenade launcher ( the improved HK AG36 ) . Ambidextrous fire controls are mounted on the trigger unit , integral with pistol grip and triggerguard , and in basic configuration are planned to deliver single shots and full auto fire . The G36 - type ambidextrous charging handle is located at the top of the receiver , below the removable carrying handle .