From Madbull, these are fully licensed Daniel Defense 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.
At 16 inches, this barrel is just a touch longer then an M4 (1.5 inches); it adds a longer looking barrel to give the M4 a more ranged carbine look like a recon rifle but functionally it does not really offer much increase in performance besides a few fps and a slightly tighter cone of fire (assuming your inner barrel is making use of that extra 1.5 inches).
The Madbull DD series of outer barrels has 3 different variants of 16 inch barrels, for the most part the only really differ in stylistic design with their pattern of varying thickness throughout the length is different in each case. You will have to take a good look at all 3 and see for yourself which pattern suits your tastes more. The skinny Government Profile, the broader Lightweight Profile or the chunky M4 Profile.