Just throwing in my experience with Mike Rock barrels. I have [2] 24" .308 5R barrels and they both average 150fps faster than the Remington factory 24" tubes with Federal 168 GMM, this is at 75 degrees and the chronograph set at 15' from the muzzle [I am running 2720 fps with this load]. I have not fired my 18" Mike Rock SPR barrel through a chronograph so I don't know what it is running, but it sure is easier to clean than a factory barrel. He has both cut rifled and button rifled AR blanks, buttoned runs around $270 chambered and cut rifled about $425 chambered.
Mike