For your purposes I'd recommend a scope in the 1-4 or 3-9 range, depending on what you like.There are many good choices in either range.My rec would be the Burris XTR in 1-4, or the Leupold mark series in 3-9 ( they just came out).
Barrel length has nothing to do with accuracy, only affects velocity which gives you more options as far as distance.Yes, you can make 700 yd hits with a 16" barrel, but you're not gonna do more then make holes in paper....

Would a 3-9 scope be enough for the 700yrd shots?

The AR-15/M16 was never intended to be a "sniper" rifle.  Specs on the M16A1 gave it a maximum effective range of 460 meters using 55-gr. M193 ball ammo.  In the USMC we use to qualify at up to 500 meters with iron sights ... and at that distance, an Echo target was pretty much swallowed up by the front sight post.  



With that said, 16", 20", or 24" barrels are all going to have about the same amount of accuracy if you are using optics - the advantage would go to the longer barreled rifles only if you are using iron sights due to a thing called "sight radius".  Being able to reach out to the incrediible distances you are taking will involve firing a much heavier/longer bullet ... probably atleast 80 grains.  To stabilize a bullet this long will probably require a 1:7 or a 1:6.5 twist, while most newer AR-15 barrels will probably have only a 1:9 twist (perfect for a 62 gr bullet).  Another problem with the heavier/longer bullets used in long distance shooting is that they have to be manually fed, because the magazine can't handle the additional length of the cartridge.



Now that I have bored you to death.  I really like the Bushnel Elite 3200 10x40 riflescope with the Mil-dot riticule.  At around $200 discount price, I think it will give you more bang-for-the-buck than ANY other riflescope.  I would also recommend mounting it to your flattop with a one-piece mount such as the Rock River Arms Hi-Rise ($47) or the Armalite mount.



I know that many will probably find fault with this old Marine's recommendation but I can handle it.  Good Shootin'!!!!

i basically have the same setup.  a rock river 16" ss midlength 1/8.  i'm putting a 2x7 burris fullfied II tactical on it that i bought from another board member for $190.  when the burris p.e.p.r. mount starts shipping hopefully next month i'll mount it with that, those are $69.  a fairly cheap setup that will be effective.  the burris fullfield II for the most part is rated better than than a leupold vari-x II but not as good as a vari-x III.  the mount has rails on top of the rings for a mini dot.  perfect.  75gr hornady should be good out to at least 500m.  all i'm concerned with is hitting man sized targets that far.  7x should be fine for putting rounds on target.  



http://www.swfa.com/pc-5649-1108-burris-2-7x35-fullfield-ii-tactical-rifle-scope.aspx scope

http://www.swfa.com/pc-14263-2735-new-burris-ar-pepr-30mm-scope-mount.aspx mount

Thank you much for your reply

I appreciate the background info and the advice.



-Fox

I should add that a scope doesn't make a weapon more or less accurate.  The weapon's accuracy will be determined by the condition and quality of the weapon and the consistancy of the load.  You, as the shooter, will have more to do with where the bullet lands than any other factor.  



For iron sights (skip to 4 when using a riflescope):



1) Stock Weld. You need to have a consistant stock weld (where your cheek comes into contact with the stock).  A lot of shooters would place a strip of adhesive tape on the stock right where their cheek would come into contact with it.

2) Sight Alignment. The tip of the front sight post must be perfectly centered (horizontally and vertically) in the rear sight aperature.  This has a more pronounced effect than sight picture (Item 3 below).  If you are having a hard time consistantly hitting paper this is the likely culprit.

3) Sight Picture.  With the proper sight alignment, place the tip of the front sight post in the desired point of impact.  You should be focused on the front sight post so that both the rear sight aperature and the bulls-eye will be out of focus (blurry).  You can tell when someone is making the common mistake of focusing on the target because they would be breaking paper all around the bulls-eye.  We called this "chasing the bull".



While a scope will relieve you of the above responsibilities, the following needs to be followed for all shooters:



4) Verticle alignment of the rifle.  Do NOT cant your rifle to the left or the right.  It will skew your sight adjustments (windage and elevation).  This will be most noticeable at long distances. If you have a real bad time with this, I think they have a level you can attach to your rifle to help you.

5) Breathing control.  Take in a breath, slowly release half of it, and take your shot.  Be patient, if you can't get a steady shot, put the weapon on safe and relax.

6) Trigger squeeze.  Take up slack and slowly squeeze the trigger until the rifle fires.  I have seen people wait until they have perfect sight picture and quickly jerk the trigger before they lose it.  You can see evidence of this by a very pronounced vertical impact pattern or worse yet, shooting over the target.

7) Let the rifle surprize you when it goes off.  Some people anticipate the weapon discharging and flinch or throw their shoulder into the rifle.  We called this "bucking" ... it will affect your accuracy as much as poor sight alignment with the round hitting the dirt in front of the target as the final result.



Learn how to use and adjust your sling to help keep you in a tight (and sometimes painful) shooting position.  Mark the keeper on your sling so that you can easily return it to the same position.  Consistancy is the name of the game if you desire to become a superior marksmen.  Keep a log book detailing each shot that you take.  Note the date, time, weather, wind direction, velocity, clouds, temperature, and distance to the target.  Mark both where you think that you hit the target and where you actually hit.  If you think that you jerked the trigger on a particular round, note that.  Write down any sight adjustments. When firing off-hand (standing), your sights will naturally adopt a figure 8 pattern.  I am told this is because of the beating of your heart.  If you try to control it, you will just get the shakes and tremors ... worse than the original figure 8 problem.  When firing prone (laying down), do not rest the barrel on any rest, log, or sandbag.  Even though the bull-eye is printed flat on the target, it will cast a shadow on a sunny day - don't ask me why.  



Semper Fi

Thank you for the information. I appreciate the advice!

1-4x or 3-9x would work great.

700 meters with 5.56 / 223 is a stretch.  Some people do it, but you'll need to reload for it.  You'll need a 1 in 7 twist barrel and the big 80-90 grain bullets.  You'll also have to single load them because they won't fit in a magazine.

For that kind of shooting, you'd be better served by 6.5 grendel, assuming you're sticking with the AR-15 platform.

My line of thinking as well - I just ordered a Bushnell Elite 3200 10X40 with a Burris P.E.P.R. one piece mount.  

Bushnell Elite 3200

Burris P.E.P.R.

Let me know how the P.E.P.R. works for you ... I am unfamiliar with this mount.