From Madbull, these are fully licensed Noveske style Aluminium outer barrels. Tough, light and high quality; Madbull makes some of the best outer barrels around and they offer them in a broad variety of lengths. A standard M16 has a 20 inch barrel while the standard M4A1 has a 14.5 inch barrel but the beauty of the M16 family is their ability to modify the length with a lot more options then that.
This 16 Inch (418mm) barrel is dubbed the 'Recon' barrel as its slight barrel increase affords the real M4A1 slightly better range and accuracy. In Airsoft however, less than 2 inches of gain over a standard barrel length does not really make much difference besides the aesthetic. If you want to style your custom weapon to look like some sort of first point reaction force recon setup then fine but functionally it will not make a difference really.
This is a light grey colored barrel with a low profile black gas block and chrome silver gas tube.