MD, you are probably right. But I don't think so. I do absolutely agree that the twist should be enough to stabilize that bullet. No doubt.
I understand 1 and 2 and yes, they are not a factor here.
I cannot even come close to seeing enough detail in that target to say anything about what bullet holes are perfectly round and which ones are not. It is hard enough when looking at a target up close and in person.f If they are keyholing sure, but then most of the bullets would be missing altogether.
But one thing is for certain - unless the two groups are unique flukes of chance. And that is that accuracy as is rapidly, very rapidly, declining with distance. Those bullets are becoming ever more inaccurate while they are flying and long long after they leave the muzzle and any influence of the forearm bedding etc. If your forearm hypothesis is correct, and the rifle shoots 3MOA at 100 it the forearm can't cause it to shoot 6 MOA at 200 because by the time the bullet reaches the 100 yds line, it has long forgotten the forearm per se. It can only be less accurate if something is happening to the bullet out there in the middle of the range. That leaves just the bullet's intrinsic properties and the environmental conditions such as wind.
If wind conditions etc, do not factor in, then the group size at x00 yards should be reasonably x times the group size at 100 yds. No news to you there, but this is dramatically larger. I would doubt that any environmental variables (e.g., wind) changed sufficiently to cause that increase in group size above this expectation or the OP would have mentioned that.
So, it's got to be intrinsic to the bullet itself. And that means stability, regardless of what the numbers say. I disagree that you can launch a wobbly bullet and expect it to be stably wobbly such that the "cone of fire" or MOA of accuracy would be maintained. It is knuckleballing out there and when it does that it's shedding velocity fast (thus the huge drop from the expected center of the group). And the big increase in MOA group size.
I don't know how the forearm would cause that group to drop like that, nor how it would induce a larger MOA. I just don't see a mechanism. So, I'm sticking with my story - besides, I've been there, done that too many times.
But maybe you are right. Maybe we will find out. It would be interesting to know.
