I've heard good things about the barrels Green Mountain makes for Black Powder, and apparently they are now making barrels for ARs in 16" and 20" lengths.
Price is damn good for a Chrome Lined Barrel at $129. M4 Extensions, threaded, etc. But I haven't found many reviews about them.
http://www.gmriflebarrel.com/catalog.aspx?catid=ar-15%20/%20m16%20/%20m4%20%20assault%20rifle%20barrels
Has anyone here ever used one on an AR?