"Maybe instability and accuracy aren't related?"
I believe I reiterated that in another thread on this subject and gave an example of such.
We can have very stable poor quality bullets and poor accuracy.
We can have poor stability with quality bullets and what appears to be "good" accuracy at a given range generally at a relatively short such as 50 or 100 yards. Frank's target example demonstrates that at 200 yards his bullets were losing stability and wobbling. While the accuracy/precision was also going south the bullets were probably not yet completely tumbling but had severe yaw and/or wobble.
If a bullet is unstable at exit from the muzzle with there will be no accuracy or precision. Even at a very short range such as 25 yards with such we can completely miss a standard size target or the bullets can go through sideways (keyholing).
If a quality bullet is marginally stable there can be accuracy even through the point (short span of range) where the bullet begins wobbling. The bullet will still may travel some distance before the nutants overcome the dynamic stability of the bullet and it completely tumbles. Accuracy/precision will get progressively worse as the range increases.
A marginally stable quality bullet may exhibit stability by making round holes in a target but give poor accuracy/precision at shorter ranges because the bullets have not yet begun to wobble.
There are other things than stability or loss of stability that affect the flight of the bullet causing inaccuracy. Some may not want to believe that but it is fact.
"Quality bullet" refers to bullets having the center of mass and center of form coinciding with the axis of the bullet.
Concealment is not cover.........