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 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 then an M16s 20 inch barrel (unless you get a special cylinder but thats a whole other can of worms).