Many thanks for the information, gentlemen.
I notice that the Steyr AUG/HBAR 5.56mm light support weapon has a 24.45" barrel, while the L86A1 (SA80 LSW) has a 25.4" one, but other MGs seem to settle for 20". The bullpup layouts of the Steyr and SA80 help to keep them compact, of course.
It does put an interesting slant on that table in the FAQs showing the minimum fragmentation distances of NATO ammo from different barrel lengths, which are frighteningly short for the short-barrelled carbines now being favoured. Presumably, these bullpup LSWs remain effective out to longer ranges than even the standard 20" barrel guns.
Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/