The MR-C does not appear to have ever been anything more than a concept model designed by Crye Associates, at the time better known for developing the Multicam camouflage pattern. While their site listed the client as "US Army," the weapon never appeared in any Army-related contests featuring assault rifles.
The weapon's statistics are effectively fictional as there was never a functional MR-C, nor is there any evidence of design work to that effect. Crye's own information describes a weapon chambered for a 6.8mm caseless round in a bullpup layout, with a large-capacity 45 or 50-round magazine and a fire rate of 800 rpm.
The MR-C was shown with its own 40mm underbarrel grenade launcher known as the "AGL" (presumably "advanced" or "attached" grenade launcher) which would presumably use standard 40mm grenades. An alternate configuration with what appears to be a magazine-fed pump-action underbarrel shotgun was also created.
The weapon shares it's over appearance from the G11 rifle, in particular the "wind up" style charging mechanism; the M4's signature quadrail.
- Most information about the MR-C is taken from the Tom Clancy's Ghost Recon series of videogames, and should not be taken as fact.