The MR-B (Modular Rifle-Bullpup) is a employed by the Ghosts. It uses 5.56mm Depleted Uranium rounds. It features Holographic Reflex Sight, a Titanium Hook, and a Disputor Shotgun.
A team of the Ghosts carried them during Operation "Take Down Overlord".
30K carried one during a mission to Chelyabinsk province, Russia.
Although the MR-B is fictitious, it appears to be based on the Magpul PDR, a prototype/concept bullpup compact weapon designed by the same designers as the ACR (Masada) to be a more potent alternative to a submachine gun that would be used primarily by support troops that would normally only carry a pistol.