“FREEFALLE7″, a Ranger Instructor, posted the above photo at AR15.com. A student, on arrival at the Swamp phase of Ranger School, showed him his M4 and said “my barrel fell off”. His punishment for not telling an instructor at the Mountain phase (the previous phase) about the broken M4 was to be given a M240, which weights 27 lbs, as a replacement.

Keith J came up with a plausible theory of how this happened

Salt water in the threads. Steel rusts. Steel in contact with aluminum. Aluminum then oxidizes, reducing the rust to iron dust. Joint remains tight until it is abused, then it just falls apart.

This all started when the barrel was installed and it was tightened a bit too much, causing the anodizing to crack

Of course this would have happened over a long period of time and use.

Thanks to Jay for the link.

Rangers from 1st Battalion, 75th Ranger Regiment, Fort Benning, G.A. stand at attention on the infield of the Lowes Motor Speedway as they are honored for their service to the nation during the Coca Cola 600 NASCAR pre-race military appreciation show in Concord, N.C. on May 24, 2009. (U.S. Army photo by Trish Harris, USASOC News Service)

A large high resolution version of the photo can be seen here.
Can anyone tell me what the device attached onto the muzzle is? I am guessing it a device to prevent the gun firing or chambering a round?

Hat Tip: Defense Tech