I've read much of the information contributed to this web site in the past year, as well as articles from the links, including the Maryland Shooters site.  

The consensus on "optimum" barrel length for a "serious" AR15 has been the 16" barrel.  Short enough for CQB but long enough to "reach out there".

Now, the reports from the Afghan sandbox include "suggestions" of lack of knockdown power from the 16" M16's; something not reported from our soldiers with the 20" rifles.

Are we changing our thinking as a result of this information?

Are there any studies measuring M855 energy out of 16" and 20" tubes at varying distances?

I don't recall any short-barrelled ARs on the line at Camp Perry in past years.

If the flag every goes up, I would prefer to be able to "reach out and touch someone" rather than waiting until I "can see the whites of their eyes".

The soldiers in Afganistan wern't using 16" barrels.

If you are shooting at Camp Perry, I wouldn't suggest using a 16" barrel. Camp Perry is all about long range accuracy and has nothing to do with combat rifles. That being said, a 16" barrel isn't nessessarily less accurate than any other barrel length.

Reaching out and touching someone is a good stradegy, however we can not always dictate how a combat senario is going to take place. The shorter carbines are a comprimise at long range. The longer rifles are a comprimise in CQB. You can't have it both ways, although a 16" barrel would be a good comprimise between the M4 and a 20" rifle. The 5.56 cartridge is a medium to short range cartridge. It is effective at ranges that the average Joe can hit under combat conditions. Combat doesn't involve benchrest shooting for the most part. Making these extreme long range shots involves a whole lot of skill or blind luck.

Last time I checked a real M4 has a 14.5in. barrel. I'm not trying to take away from your point but just for the record.

Glockdog

Airborne!!

??Camp Perry is all about long range accuracy, and has nothing to do with combat rifles?? Then why do they have all those goofy rules (in most of the matches) about the rifle having to be the same as military issue arms?? Isn't 'issue' what most of our troops involved in combat are using? There are Springfield 03 matches, Garand M1 matches; and M1A and AR15's have been the most prevelant arms used in the matches. Aren't/weren't those all "combat ' rifles?

Depends on two things.  Ammo used and typical engagment distances.  Afghanistan was not typical and there were some extended shots  (and some problems with thick jacketed M855).  Now in Iraq typical small arms engagments were at 30 yards or less! and sniper shots were RARELY over 300y.

Using a decent round (like the 77gr OTM Mk262) solves a number of problems.  You don't have to worry if your batch of M855 has the thicker jacket, and your fragmentation range is greatly increased.

If you go to the Ammo forum you'll see the results of my testing some Mk262 that Black Hills is producing.  You'll find there isn't a big muzzle velocity difference between the 16" and the 20" rifles.  Not enough to rate carrying the longer and heavier weapon.

BTW there is no such thing as 'Knock down power' and using 'energy' as a guide to terminal performance is meaningless.  For more info study the FAQ at MD-AR15.com or the Ammo-Oracle.

BTW you don't see short barrel ARs on the line at Camp Perry for several reasons.  
1) They are not allowed in Service Rifle class even though they are issued.  
2) High-Power is a game - It in no way represents real combat (either the weapons or the style of shooting needed).

Funny though last year the top scoring Marine (for their rifle qualifications) used an M4....

BTW the rules at Perry are the rifels must LOOK like military rifles - not be military rifles.  I don't any issue M16A2s with lead weights in the stock, or HBAR profile stainless barrels, fragile match triggers, or DCM free float tubes....

IMHO, the optimum barrel length for velocity, accuracy and general all around handiness is has been since 1963..... 18".  

(Check out what some of the SPR builders are now using - 18".)