I was recently in a local match called a Rattle Battle.  You fire from three positons at a standard IDPA or IPSC target. You may fire as many rounds as you can in 50 seconds. I shoot open sights and I use a sling.  I am sighted in for dead on at 50 yards.  I hold on center mass at 100 and 200 and about 5-6 inches high at 300.  We shoot 100 off-hand, 200 sitting and 300 prone.  I clean house at 100, do very well at 200 yet, lower than I should be, and at 300 only hit 2-3 / 30 and they are 15-20 inches lower than they should be.  My QUESTION - Is the sling pulling down the barrel (20 inch HBAR NM barrel non-floating) enough to cause these very low hits at 300 or is it likely that my sight picture is off shooting prone?  I bench rest the rifle to zero.  Any suggestions?

I do not think its your sling, but your zero.  When you say "lower than they should be", I think gravity is your main problem.
What I suggest is sand bag the rifle at 300 and determine your actual come-up for this range from the bench, given as you said "you are using a dead on 50 yd zero" to shoot accross the course, but holding over at 300.
Then repeat prone (again, support forend on appropriate height sand bags).
If you dont have a range adjustable rear sight, keep track of the front sight elevation clicks from your starting zero.
This is what we did between the 200, 300 and 500 yard lines during re-qual with the M16A1. That is why everyone hated the front sight post being the only way to correct for elevation range changes if one wanted to shoot point of aim =  point of impact.
I also think holding "center of mass" is much more precise and repeatable than subjectively repeating, say 6-inches of hold over, at 300.

First off....you did not mention what grain bullets you are using or your FPS however.....typically out to 300 yards with a 50 yard zero you should have been ok with what you said with either 55gr/3000fps or 69gr/2750fps.

I zero at 50yds and out to 200 I am within 1.5 inches of line of sight. at 250 I have to hold over about 3.00 inches (still dead on for an IPSC target) and at 300yds I am aiming at the center of the upper scoring zone to get 'A' zone hits in the lower scoring zone.

I shoot a nosler 55 grain BT and it is 3000fps out of the barrel

50yd '0'
100yd +1.35
150yd +1.46
200yd +.15
250yd -2.75
300yd -7.53

You may be having a problem with your sling and you will want to check that which should be fairly easy to check at 100yds.....just shoot a few groups off the bench and follow up with a few groups from prone using the sling.

captainH1,

 What ammunition are you using?  Let me know and I'll look up the trajectory for you and tell you how much its supposed to drop at 300 yards out of your 20" AR.

good suggestions.  Thanks for helping with the diagnosis.  I am shooting 55 Gr PMC.  

Thanks for your suggestions.  Shooting 55 gr PMC and it shoots in my gun about 2850-2900 fps.

55 gr PMC, good suggestion on the sling.  Just difficult for me to get to a range to check things out in urban area in which I live.

Actually, I have this sighted in with the small aperature at 100.  I use the large aperature for in close IPSC 3 gun shooting, but the small for precision at distance.  Thanks for your suggestions,.

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Captain, which apeture are you using and what range do you have the small apeture zeroed at?

You should be using the small apeture for these stationary targets at 100, 200, and the 300 yard line. The small apeture should be zeroed at 50 yards(IBZ) -2 8/3.

Consult the page below for your trajectories, which as you see POI should be about 6 inches low at 300 yards. Don't get confused. Use -2 8/3 and the small apeture at this competition. If you desire point of aim, point of impact at 300 yards, just click up 2 clicks of elevation on the rear sight to 8/3. This will put you in the center of the bullseye at 300.

[url]ttp://groups.msn.com/TheMarylandAR15ShootersSite/zerotrajectories.msnw[/url]

The ghost ring should only be used while patrolling, in low light conditions, or verifying ghost ring zero etc..., not shooting at targets.