Informed observers think this rifle a variation on the M-4, which itself is a variant of the M-16 is the gun that killed Osama bin Laden in the American raid of his Pakistan compound on May 2, 2011. Weighing in at just under six pounds, its 26-inch length (with stock collapsed) makes it easy to carry off of choppers even via rope and through cramped rooms and stairways. Its 10-inch barrel makes it less accurate than its longer-barreled brethren. But it's good to go at anything within 100 meters or so, in three-round bursts or fully automatic. Its magazine carries 30 5.56mm rounds. The MK-18 was developed at the Naval Surface Warfare Center Crane in Crane, Indiana.
For the inside story on America's military operations around the world, check out Mark Thompson's blog, Battleland: Where military intelligence is not a contradiction in terms.