This is the Heckler & Koch Infantry Automatic Rifle (IAR), which is currently being considered by Marine Corps commanders as a partial replacement for the Squad Automatic Weapon (SAW).
The AIR has gotten mixed responses. Some Marines prefer the SAW's 200-round firepower to the IAR's 30-round ability, and say that the "psychological advantage" of having a SAW in your hand would outweigh the IAR's improved accuracy. But IAR's accuracy could be preferential for commanders in Iraq and Afghanistan, who are more concerned with taking out individual targets and preventing collateral damage.