Source: http://www.dtic.mil/ndia/2003smallarms/john.ppt
Above: A M4A1 with SOPMOD package, including Rail Interface System and Trijicon 4x ACOG. The barrel length is 14.5 inches.
Below: A M4A1 with a Close Quarter Battle Receiver. The barrel length is 10.3 inches.
This image is a work of a sailor or employee of the U.S. Navy, taken or made during the course of the person's official duties. As a work of the U.S. federal government, the image is in the public domain.