I'm setting up a sniping/long range rifle on the AR-15 platform.  The project is pretty much laid out except for the barrel.  I have seen a majority of these style rifles w/ longer barrels but I'm wondering if that is neccessary.  Has anyone done a study to see at what length a 223/556 starts to lose velocity?  I know it will vary depending on the load/manufacterer but I'm wondering if there is a sweet spot in there somewhere.  I've heard that an 18" barrel is sufficiant to obtain max velocity but I have not found any data to back that up.  Any help would be greatly appreciated.

ETA:  Kinda ironic this is post 223.

An 18" barrel is not sufficient to obtain maximum velocity.  I have heard that in a 22LR rimfire,  an 18" barrel loses velocity over a 16" barrel, but I have not verified this on my own.  Many people believe that 18" barreled .223's have the best ballistic efficiency, but this is not saying the same thing as maximum velocity.  Most reloading manual data is written for 24" test barrels.  Krieger will make you an Ar-15 barrel all the way out to 28"!!  ...Size matters!