Oh so many questions, so little time to ask them.
as I see it, 3 shot groups get you good prospective loads, 5 shot groups rule out loads that seemed good but aren't, 10 shot groups normally tell you when you have a solidly good load. strings of 3 shot or 5 shot or 10 shot groups are the only way to test out the load to see if it is a good/great load.
I have heard so many ways of choosing a good load, one that I laugh at is one a friend uses and he is a good shooter.
My questions start with: thin barrel or heavier barrel?
Was the sub 0.4" group shot with a hot barrel or was it cool/cold?
I have a rifle that shoots awesome (sub 0.5") with a warm barrel and crappy (1.25") with a cold one. that is my play rifle and it makes some people scratch their heads. the real problem is it is not a varmint/high round gun. it's a M-70, featherweight, carbine (16.5" barreled) 270 Win, light pack rifle.. AKA my go to deer rifle.
did you use a ladder loading method? (I really hate this method- I never can come out with even a mediocre load)
I had a Winchester 70 coyote and Browning something in 223 WSM that hated loads much above starting loads. you might have one of those rifles.