Or so the saying goes reference accurate cast bullet shooting.
Couple of interesting things being batted around. Yawing instability increases should show in increasing BCs. Could be that the test design sensitivity or equipment is adequate to show increased group size but inadequate to show changes in BC correlating to yawing or other bullet instability. Very interesing stuff!
If on the other hand RPM increases are inducing larger spiral bullet paths to target while the bullet is still aerodynamically efficient and pointed in the right direction... then BC changes probably wouldn't show up and would be very, very small. If that is the theory behind the RPM threshold then it seems something like a "Rotationally Induced Precessional or Perturbational, Escalating, Harmonic Magnus Effect" is in play. Sorry, best terminology I could come up with to explain the theory. Whew! that is getting into some serious and complex physics. Without equally serious test equipment and expert help I wouldn't have a clue as to how to tackle explaining it or testing the theory.
BTW agree with PatI.... even if bullet gyroscopic precessional reduction or "settling down" or "going to sleep" or whatever it's called happens between muzzle and target--> there is no way relative group sizes will get better with increasing distance. The best one could hope for is a trajectory dispersion curve changing from curvilinear to more nearly linear.
NO expert here just shooting out some more ideas :mrgreen: