Nobade
What alloy were you using with the CMs that broke in half? I've only shot about a hundred of them now and only 40 were at 2000 - 2500 fps. started getting keyholing around 1950 - 2000 fps. I never go any indication of the hits on target or double impacts in the berm that the bullets had broke apart. I was using 80/20 linotype/lead with a BHN of 19-20. I can easily see how what you say could happen. There is a lot of mass in that long unsupported nose and not really much diameter at the bottom of the lube grooves.
Given the inertia of the nose mass resisting the fast spin the nose could definitely twist apart. This is one of the things that no doubt happens to the long unsupported noses of more pointed cast bullets, especially those with a scraper groove like the 311413. They may not twist in two but the bend to one side and/or get distorted which unbalanced them. The faster the twist with a given acceleration rate the more the damage. Perhaps this is what goodsteel and others are referring to by faster twists being "harder" on bullets than slow twists? If so we are in agreement as I was thinking of skidding or stripping. The faster the twist at a given acceleration rate the more damage/unbalancing will happen from the torque to the bullet and overcoming the inertia of the bullet.
Then if we think about what is happening to the bullet in that way the equal amount of unbalancing required for the cast bullet will happen at a lower velocity in a fast twist than in a slow twist. The torque required for equal unbalancing of the bullet is produced by the same RPM in both. Equal RPM occurs at a lower velocity in the fast twist than it does in the slow twist. Amazing!
Thanks for the train of thought Nobade, we may have made things a lot clearer regarding what is happening during the internal ballistics phase.
Larry Gibson