I think even the American Rifleman article about Don mentions smaller groups.
FWIW my best 600 yard 4-shot group is 1.576 (witnessed and signed).
I was just playing around that day to see how my 6.5-284 XP would group.
Roy was spotting from a closer distance (safely) and never mentioned what the group size was doing.
3-shot groups @ 500 meters could go even lower if your goal was to see how small you could get them.
It is just a matter of spending time and getting the right conditions.
Of course you need a certain level of ability from both the shooter and the SP. SP's are now shooting at 1000 yards under an IBS format and putting 5 shots for record score under 10 inches.
My 11 year old this past summer shot a 3-shot group and a 4-shot group w/a 6.5-284 MOA Maximum using a portable bench just East of pueblo, CO that was right @ the 10 inch mark himself (good conditions and a Bower rest sure didn't hurt :-D ) We didn't even measure the two groups. There was at least 4 other adults there at the time he was shooting besides myself.
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