Glenn, using handloads through my No1 MkIII* HT I once put 5 rounds in 7" at 700yds. The object of the exercise was to compare my handloads to the best surplus, so I followed up with a few Greek HXP MkVII and found the MPOI to be the same, but the group opened out to about 14".
Perhaps have a look into what the rifle club shooters were doing with heavy barrel SMLE post WW2 with aperture sights. I just did a little searching, but couldn't come up with enough information with any one set of results eg, range, target dimensions, scoring, possible score etc. Maybe some others here have something concrete in this area. Really looking for possibles shot at 600+ meters. I think you will find it was fairly common, making the claims made above credible for a practiced sniper. I'll keep looking and get back with anything I find.