Brass I picked up from the Burris Optics Team Challenge that was just held in Douglas, WY. I RO'd, so I saw all teams shoot both a Precision and an Assault portion. I saw 16" to 24" barrels from .223 to .308. AR15 patterns edged the .308 patterns. .223 was the most common for sure in the carbines, probably 18" or 20" barrels. A few teams ran matched ballistic guns in 6.5CM or .308 and did really well with them in terms of time management and common calls on wind and elevation.
An 18" barreled AR15 with 5.56 pressures and 69 to 77 grain bullets seems to be the most typical based on what I have seen in the matches I have shot or worked with both guns.
Hope that helps.