All Daniel Defense cold hammer forged barrels are made from start to finish in our state-of-the-art facility located in Black Creek, GA.In keeping with our heritage of not cutting corners, all cold hammer forged barrels feature a forged chamber and heavy phosphate exterior coating. They are all MP tested on-site and are available in multiple configurations. These barrels will prove themselves to be more accurate and last longer than a barrel that has cut or button rifling."

Madbull Daniel Defense licensed Outer Barrel 12.5" (Lightweight Carbine/ Steel)
Authentic Daniel Defense markings
Machined steel for realistic weight (heavier than aluminum barrel)
Phosphate finish, like real steel
Shortened CQB length 300mm inner barrel recommended
Includes 14mm CCW Threading for attachments

High quality Phosphate finished steel; 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 than that.
At a full M16 length of 20 inches, this is about as long as a barrel can be before you run into problems. Not just unwieldy lengths but if inner barrels stray to far beyond 510mm they run into volumetric problems as the standard Version 2 gearbox cylinder lacks the internal volume to fill such a large inner barrel and so starts to suffer from negative pressure causing a drop in performance.
So, in situations where you want maximum pressure to FPS efficiency and the tightest cone of fire you can get out of an AR platform then you really need to look no further than an M16s 20 inch barrel- unless you get a special bore up cylinder.