Closeup of Colt extended feed ramps, which they have added to ALL their carbines, including the M4 series.  The feed ramps are extended from the barrel extension onto the upper receiver.  "Standard" feed ramps end at the edge of the barrel extension.

The rifle on the left has the extended ramps on both the barrel extension and upper receiver, as intended.  The rifle on the right has the extended ramps on the barrel extension (i.e., cut at a shallower angle and designed to match up with a 'cut' upper receiver), but a standard upper receiver with no feed ramp extensions.

-Troy