I've re-tested with a new powder and a new primer, because although the previous test of 14 March looked ok on paper, it did not shoot ok at all between 600 and 1000 meters.
This is Norma URP with CCI250 and Nosler CC 155 grainers.
The previous of 14 March was N150 with Fiocchi primers, same bullet.
I pick load 2 as representing the scatter node, bacause it has the highest vertical and horizontal dispersion. It is 70.5 grains.
Group 5 is at 71.4 grains, which is 0.1 grains away from 1.5% of group 2.
Putting these side by side tells me that the combination of URP and CI250's performs much better than N150 and Fiocchi LRP.
Your sample sizes are a tad small for definitive conclusions.m
I must say, its hard to explain, but some powders with similar burn rates to others just work so much better in some guns than others.
A chamber cast and careful measurement of the cast might provide some answers ?
Very small variables can play a big role