This is a standard M4 barrel with the lighter Government profile. Many M4 and M4A1 rifles used this barrel. The other barrel used was a heavier profile, called the SOCOM barrel, which was more appropriate to hold the M203 grenade launcher. As the barrel originally came from the factory with an A-frame Front Sight Base, it has been removed, and there are grooves where the pins were drilled to attach the FSB. Charlie's has chemically treated those areas of steel to be black and resist corrosion.
Colt
Daniel Defense