After firing 6,000 rounds through ten M4s in a dust chamber at the Army's Aberdeen test center in Maryland this fall, the weapons experienced a total of 863 minor stoppages and 19 that would have required the armorer to fix the problem. Stacked up against the M4 during the side-by-side tests were two other weapons popular with special operations forces, including the Heckler and Koch 416 and the FN USA Special Operations Combat Assault Rifle, or Mk16.

Another carbine involved in the tests that had been rejected by the Army two years ago, the H&K XM8, came out the winner, with a total of 116 minor stoppages and 11 major ones. The Mk16 experienced a total of 226 stoppages, the 416 had 233.

http://www.defensetech.org/archives/003909.html

It is no surprise that the Army rejected a better weapon. They have done the same with body armor and other troop accouterments. The latest that I heard is that it had something to do with getting a job with the manufacturer after the end of some of their careers. This is not to say that individuals within the establishment have only the best intentions in mind when it concerns the troops but the Army has had a long history of procurement problems.

This is a few years old but interesting in its own way:

http://anarchangel.blogspot.com/2006/11/top-ten-battle-rifles-huh.html

This one is more directly tied:

http://www.armytimes.com/news/2007/02/atCarbine070219/

Ive been seeing this test posted on all of the forums in the last few days... One thing to keep in mind is that over 200 of the malfunctions were contributed to the magazines, so with out those it comes out to just over a 1% malfunction rate... No weapon will ever be perfect, the individual soldier or Marine is going to be responsible for keeping his weapon clean and lubed and his junior leadership is there to make sure its getting done... Personally i have yet to have an A2 or M4 hiccup once that wasnt caused by faulty mags... And when it all boils down to it, its still a matter of each person taking care of their weapon and in the case of the M4 being trained to perform an IAD or SPORTS without even thinking about it and get back into the fight