The 300m ISSF target is made for iron sights shooting. It's a pretty neat target actually - the ten-ring is 100mm in diameter, the nine-ring is 200mm in diameter, etc.... i.e. all rings are multiples of 100mm, a nice round metric target. I seem to recall hearing that this has been the 300m target "forever" (since the 1920s?).
http://www.shootingwiki.org/index.php?title=ISSF_Target_300M_Rifle
As an iron sight target, it is pretty darn challenging (even moreso when you realize that in ISSF 300m shooting they also shoot at it from the standing and kneeling positions too -- yikes!). I know that a 600 has been fired on the ISSF for 60 shots prone, by a Norwegian guy. I don't know if he is the only person to have ever done that, or if his accomplishment is "merely" rare.
As an F-Class target you would think that it is almost trivial; 100mm at 300m means that the 10-ring is 1.15MOA in size, and it is pretty straightforward to get a first-rate F-Class rifle that shoots an honest half MOA.
And yet, you don't see very many 600s being shot on the 300m ISSF target by F-Class shooters. It does happen from time to time, it certainly is doable, but it's a most respectable achievement by a top shooter in order to be able to claim that accomplishment.
To not shoot a 600 on the ISSF 300m target takes only one mistake, and you have 60 "opportunities" to make that mistake. The mistake could be breaking a bad shot, perhaps your attention wandered. It could be failing to keep your group well-centred (your 1/2 MOA group does fit in the 1.15 MOA ten-ring, but that only leaves 0.3MOA between the edge of a perfectly-centred group and the ten-ring edge). It could be missing a wind change (not a huge factor at 300m, but a wind change can easily move your bullet a quarter of a minute, and it's not a terribly big wind change to move you a half minute).