Robinson Armament XCR

The Robinson Armament Co. XCR is a multi-caliber, gas piston weapon system developed by Robinson Armament Co. for U.S. Special Operations Command (SOCOM) to satisfy the requirements of the SOF Combat Assault Rifle, or SCAR competition, but was disqualified on a technicality due to late delivery of blank firing adapters. Robinson Armament continued development and the XCR is now being offered to law enforcement, the military and general public. Deliveries of the rifle began in mid-2006. The XCR was displayed to U.S. Army officials during an invitation-only Industry Day on November 13, 2008. The goal of the Industry Day was to review current carbine technology prior to writing formal requirements for a future replacement for the M4 Carbine.
Overview.
The XCR utilizes a heavy duty bolt and extractor connected to a long stroke type gas piston. The bolt and extractor are designed and patented by Robinson Arms, and promoted as offering higher performance over eight lug M16/M4 type bolts. Other features include a folding stock which eases storage space consumption and deploying from a vehicle, in addition to telescoping M4 Carbine style stocks for those who prefer length of pull adjustments; it also has a monolithic top Picatinny rail with side and under-barrel rails, and forward assist integrated into left-side charging handle.
Caliber conversions.
The XCR is a multi-caliber weapon system. The base rifle is chambered in 5.56×45mm NATO. Conversions to other calibers and barrel lengths can be performed in 2 to 3 minutes without the need of a trained armorer, and using only a standard hex wrench, and consist of changing to the appropriately chambered barrel and bolt, replacing brass deflector (optional) and insertion of appropriate magazine. .300 AAC Blackout, 6.8mm Remington SPC, and 7.62×39mm are the current conversion kits available. 6.5mm Grendel was slated for release in 2009, but is currently on hold due to technical issues. The barrel retention design of the XCR is seen as a weak point to the design (when compared to the design of the Magpul Masada / Bushmaster ACR in particular), as the retention bolt can back out slightly, potentially causing POI shift and function issues. 
In addition to conversion kits, complete factory guns in the alternate calibers are also sold.
Operating mechanism.
The XCR employs a Kalashnikov type, gas operated action with the bolt carrier attached to a long-stroke gas piston; the gas chamber is located above the barrel.
The proprietary bolt is a three-lug design that locks onto the barrel extension leaving the upper receiver unstressed. There is no need to check the headspace when changing barrels. A steel fixed ejector is attached to the inside of the receiver, held in place by two bolts. The manufacturer claims this design provides stronger ejection than what is offered on the AR-15’s spring-loaded ejector design. The ejection pattern is to the two o'clock position of the operator, with an optimum distance of 15 ft to 20 ft depending on ammunition type and gas setting.
Magazines are STANAG 4179 type.
Construction.
The upper receiver is made from anodized aluminum forgings, and is non-stressed. It features a non-reciprocating charging handle on left side which also acts as a forward assist.
The quad rail system is integrated into the upper receiver and compatible with most 1913 Picatinny devices. The top rail is monolithic and 17” long, providing a rigid, uninterrupted plane for mounting optics and BUIS (Backup Iron Sights); the 3, 6, and 9 O'clock Position Rails are 8" long.
History and variants.
The XCR was designed in 2004 by Alex J. Robinson of Robinson Armament Co. Production of the XCR-L variant of the rifle began in mid-2006.
The XCR-L is currently available in 5.56×45mm NATO, 6.8mm Remington SPC and 7.62×39mm calibers. Each of these calibers is available in kit form for converting an existing rifle to one of the other calibers.
The XCR-M .308 was officially confirmed via Robinson Arms email circulation to a public reveal at SHOT show 2011.
The 6.8mm Remington SPC variant began shipping in November 2007.
The 7.62×39mm rifles and conversion kits began shipping in July 2008.
In August 2009 Robinson Arms began shipping 3 different sized upper receivers enabling PDW and pistol style variants:
Variants are also available in "California" versions which are limited to meet the more restrictive State of California firearms laws.
Robinson Armament also produced an 18.6" barrel version for the Canadian market. The XCR rifles intended for the Canadian market were shipped with the FAST stock (fully adjustable stock), although aftermarket stocks are available as an accessory. These rifles shipped with a single magazine pinned to accept only five rounds. On May 1, 2020, the XCR was reclassified as a Prohibited Firearm and is no longer legal to import or sell in Canada.
Since its introduction in 2006, components of the XCR have been updated. Most of these enhancements are available to existing XCR owners.