In the September issue of GunWorld magazine, Massad Ayoob tested a 16.5 inch bbled Ruger rifle against a 20 inch version.  The shorter bbl averaged 80fps higher than the 20 inch version.  
  Ayoob explains this phenomena by pointing out that many new .223's are loaded with fast burning powders that are designed for shorter bbled rifles.  With this kind of ammunition, the longer bbl only serves to create drag on the bullet, hence the lower velocity.  I thought this was very interesting, as I have always figured that a longer bbl equals more velocity.  What a shocker!

It could be true, given a faster burning powder.  Reloading could tailor the load to the bbl with the choice of powder.  A slower burning powder would do better in a longer bbl.  

Someone correct me if I'm wrong, but I figure the optimum load for a particular bbl would be one that gets little to no flash.  I would figure that a flash comes from unburned powder burning outside the bbl, and if you have that, you're using more powder than you need.  

I get a big flamethrower flash out of my 16" bbl with S&B ammo, I'll bet it was designed for a 30" bbl. [:)]

If I hold a Bic lighter to my butt after eating Taco Bell, my butt is a flame thrower.

And so, Dear Reader, the technical quality of the thread was all downhill from that point...

I was wondering just what kind of ammunition he was referring to, as I only have Q3131a for my 16" bbl AR

My ammunition is four chalupas drenched in fire sauce.

Several months ago there was an article in Guns and Weapons for mil. and LE on M2 Corp. new 5.56 subguns and it mentioned new powders in R&D that were being developed specifically for SBR's.  I can look up the particular issue if you want.

CIB, could you look it up for me, I've been looking for that for a while.

I beleive the powders mentioned in the article reached their pressure peak extremely quickly, which produced sufficient pressure for 3000fps out of a 7" or 7.5" baarel.  Significantly lower velocities would be acheived in longer barrels because pressure would begin to fall before the bullet exits the muzzle, which might serve to explain Mr. Ayoob's findings.