Not at all sure if this helps but I went through similar angst with my 7x65r. 3.5 inch groups at 100m. I have a 2-12x42 VX6 mounted on it.

I thought maybe the rifle was regulated at 150m or 200m. I shot groups at 150m and the groups where around 2.5 inches for 2R and 2L. At 200m I was about 2 inches. I did not have a 250m stand but at 300m it was about a 4.5 inch 4 shot group.

I mostly hunt bushbuck and pigs with this and a shoulder hold just on the crease half way up the body is lethal out to 300m.

With your scope I would try shooting longer distance and see what happens.