I've ever understood the LG rpm business, and I've tried.
RPM = mv (fps) X 720)/twist (") = (mv/twist) X 720. Since stability is a function of twist and mv, as is rpm; then stability is a function of rpm-all for a given bullet. But this is a tautology, nothing has been brought to the table.
Here's where I am on twist and stability:
TWIST, MV, STABILITY AND ACCURACY SUMMARY
As mv falls; bullets begin to tip and accuracy remains reasonable; then, as mv continues to fall, bullets print sideways and accuracy may remain reasonable
A 68 gr Hornady BTHP .22, .983”, Greenhill min. twist = 7.7”, shoots accurately in a Stevens 223 M200, 9” twist bbl., down to 1920 fps, where it begins to tip. Reasonable accuracy continues to 1490 fps.
Same bullet, Shilen 22-250, 9” twist, begins to tip at 1713 fps, reasonable accuracy continues 1340 fps, where bullets print sideways and accuracy is still reasonable.
(More examples in the EXCEL workbook)
So, the definition of “stability” looks to be sorta fuzzy, tipping bullets and sideways bullets can be accurate, and reasonable accuracy extends over a ~400 fps window.
Greenhill is about short, fat bullets. As bullets lengthen, get pointy, and acquire boattails, the Greenhill solution increases, twist # rises.
Greenhill is about bullet density. As alloy density decreases, as grease grooves increase effective size, the Greenhill solution decreases, twist # falls.
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Still; working on it, shooting 2-3 times a week.