The hypothesis I would submit is the following:
You are using Miller Stability, which at the end of the day, is nothing more than a model. It might be a really, really good model for 5.56 nato and tell you exactly what is going ot happen before the first shot is fired. Ditto for any similar cartridge to the military standard, as I believe Miller was developed for the military, yes? Thus for example, most bottleneck rifle cartridges would model very well.
I agree that most of the calculators are using either Miller's improved Greenhill formula or a derivative close to it. However I stumbled across the below that appears to take some additional information in to account and shows it to be much lower than the others however it still indicates it "should" be stable.
This may not apply so well to pistol. In fact, if you want even more fun, model the 460 S&W. You can get results of stability >12, yet the bullet will tumble.
I don't have access to one but would be willing to use a donor for a year or two for testing
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Last thing I will suggest is that powder is NOT taken into account in these models. Therefore, the model doens't care if you spin your bullet up to 1200 fps using H110 or AA#9, the stability is the same. Yet, at least for MY gun, I can get round holes with H110, and ovals with AA#9, and that is with a bullet I GUARANTEE is stable at that speed, and nicely accurate with H110 to boot.
In short, I question the model.
I agree the models do not look at every parameter that could influence bullet stability that was why I mentioned them in the original post. The many different outcomes we receive with seemingly benign/parallel changes is what makes our hobby so frustrating/much fun.
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I strongly doubt the recoil model, because the barrel flip velocity is going to be HUGELY slower than bullet speed across one inch of bearing surface. I'd want to see a calculation of the actual flip during bullet exit.
I understand your doubts as that is where I started but then started thinking about how much mass there was and the potential leverage it would have at 1.3" hanging out of the barrel to start the tipping upon barrel vertical acceleration. At that point the high pressure gases would be hitting the base at an angle and could create additional disruption. I know this is speculative at best and pulling it out of my nether regions (likely) at worst.
Thank you for taking the time to consider this as I know in the end we may never know. I just can't leave"it just is" alone.