If you go back to Greenhill and his calculations, the critical factors are Length of bullet and the density of the air.

That second one was a prominent arguing point with the adoption of the M-16 and the M193, 55gn bullet. Original twist was 1: 14" twist. The 'Arctic" tests in cold, DENSER air indicated quite strongly that a change to 1:12" twist would be a good idea.

The 55gn, boat-tailed bullet at the "standardized" muzzle velocity was adequately stabilized in a 1:14" twist barrel everywhere except the deep arctic. The "adequate" stabilization IN AIR meant that as soon as that fast-moving bullet entered a different medium, ( like a big bag of red, oxygenated salt water), there was a radical change in the mathematical calculations and at the rotation provided by the 1:14" twist, the spin rate was insufficient to maintain stability after entering the 'new medium". THAT is the reason for the reports of "spectacular" wounds generated by the nascent M-16. Basically, the bullet started to YAW and in doing so, would start to tumble and / or break in two.

This phenomenon is not unknown in other "service" bullets, like the Mk7 ,303, for instance, where impact with bone would cause serious bone damage AND what is generally called "tumbling" as it kept on going.

An aside on matters of weight (mass):

For some years, competitors in the US military used "modified" M-16(A1) platforms in the "Match Rifle" division. One of the best bullets "across the course" was the Sierra 63gn SP spitzer, a slightly "dumpy-looking" flat-based bullet that is pretty much the same OAL as the M-193 bullet. Perfectly stable in a 1:12" barrel, unlike the one grain LIGHTER M-855 / SS-109 bullet. Why? the M-855 is LONGER for weight because it has a boat-tail and a long ogive AND has a composite lead / steel core. The M-16A2 went to a 1:7" twist. The M-855 will cheerfully stabilize in a 20 inch, 1:10" barrel, so why 1:7"?

The "experts" could not design a tracer that was of any real use until they came up with the FN L-110 / M856 tracer bullet which is VERY long, just because it has to contain enough trace compound to be still burning and visible, over 600M downrange. On the two-way rifle-range, riflemen often use tracer for target indication, hence the "necessity" of the fast twist in the standard service rifle.

If you look at "match rifles" in .308Win, you will find that many are running slightly tighter twists than the "standard 1:12". The slinky "Very Low Drag" bullets are "slinky" because, for their weight, they have longer ogives and boat-tails to maintain aerodynamic performance EXTREME range. A long bullet with a short bearing surface is a tricky thing to stabilize.
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