maximum effective barrel length for an AR?

[newshot]

I should start by saying I currently have a very limited understanding of how barrel length and twist effect accuracy. I know at a certain point barrel length stops assisting with increased velocity and accuracy. What is that point with a 556/223 AR-15? Is it effected by the twist rate of the barrel? What would be the most accurate combination (all things being equal with the ammo being used) if the only goal is to get as close to a quarter as you can at 300 meters? Is this an opinion question, or is there a correct answer?

I should also mention I know there are more accurate platforms than the AR and that there are better calibers for 300m shooting then 223, but that does not pertain to my question.

Thank you!

 

[J996]

20" Hbar 1/7
This will give you the needed muzzle velocity for distance and twist rate for the heavier bullets.

 

[VanIsler]

In the Surefire Combat Tactics magazine they tested a barrel from 24" to 5" by cutting it down one inch at a time. Their results were that velocity was maximum from the 20" barrel and longer barrels had lower velocity. They stated that the 5.56mm Nato cartridge was designed for a 20" barrel. All the charts I have read on the internet show the velocity increasing up to and including 24", so I am not sure if their results are always true or if the tables I have seen are all theoretical and don't represent reality.

Personally for 5.56 I wouldn't go longer than 20" if you need more go up to .308/7.62 Nato.