No difference in accuracy between a 16" or a 20" of the same quality shooting the same ammo. Velocity would be slightly lower in the 16".
Length does not effect accuracy. Barrel design, twist rate/bullet match, full floating the barrel or not, chrome lined or non chrome lined, ammo quality, 223, 5.56 or "other" chamber/bullet match, optics and the nut behind the trigger.
Heavier barrels are not actually more accurate but they will not be effected by the barrel heating up as fast as a pencil barrel.
You need to match your bullets to the barrels twist rate for best accuracy. Usually light bullets like a 40gr varmint bullet isn't a good idea in a fast twist rate such as a 1 in 7.
A full floated AR barrel can shoot more accurately than one not free floated if shooting from a bench, bi-pod, using a sling or in any way that places pressure on the forearm or barrel.
Chrome lined barrels can be accurate but usually they are not as accurate as non chrome lined. There are "target" barrels that are made to tighter tolerances, lapped to smooth the machining of the bore and whatever else the each manufacturer does to make them are the best choice for accuracy. Chrome lined barrels are made for blasting and for ease of cleaning but I have had a few that were very accurate.
Good quality factory ammo or well made reloads will almost always shoot more accurately than military type ammo.
In general a 223 or a Wylde chamber will shoot match bullets more accurately than a 5.56 chamber.
If you want to read more on that here's a link to a short explanation of chambers in the AR.
With optics and accuracy, you get what you pay for. You will never shoot as accurate as your setup is capable of with cheap chinese $100 optics. I know some think they can, fine believe it if you want.
Same goes for the mount.
Sorry for the long post, nothing on TV, feel free to have a different opinion, these are mine.