Originally Posted by
Infidel
What is the intended purpose of the rifle and what twist rates are you looking at? While there is a change in velocity between the 16" and the 20", for the average shooter and in average circumstances the accuracy difference would be negligable. Your choice of bullet would make more difference as any decent AR should be 1-2 MOA. If you want a tack driver to hit at 800 yds then yeah go with the 20." If this is a home defense/SHTF/bug out rifle, go with a 16" for the added maneuverabilitly and in that situation, you should not have to worry about engaging people greater than 300 yds.
So I ask again, what is your intended purpose for you rifle, as that will help answer your question.