Marine friends informed me that it’s not the size of the round, it’s the guy pulling the trigger. For close-in urban work, they prefer the M4 to long guns but the old M14 and the bolt-action M40, both in 7.62x51 NATO, are great to reach out and touch someone.
I think you’re on to something in your last sentence. The SA80 has been re-worked by H&K but MARSOC Marines I know say it’s still not a very robust design.
I think this is needed for the windy canyons of Afghnaistan:
http://en.wikipedia.org/wiki/File:Soldiers_patrol_Waygul_Valley_2009.jpg
http://en.wikipedia.org/wiki/File:Army_M14_Sage_Stock.jpg
