Of course, those were the “good-old-days o black powder and lead bullets and comparatively “sedate” muzzle velocities.

It boils down to bullet diameter(D), length(L), specific gravity (SG) and velocity(V).

Note that “length” does not rate a mention. With lad bullets with decidedly un-streamlined shapes, that was not necessary.

With modern, stream-lined, jacketed bullets all that changed. A modern classic demonstration of that is with the 5.57 x 45 / .223Rem.

Back when it was first introduced, the original Armalite round was loaded with a 55gn, slightly-boat-tailed bullet, fired through a barrel with a 1:14″ twist. Most folk have heard the story about that combination being both accurate and spectacularly “effective” in the hot tropics, but all over the place in the Arctic. AIR DENSITY was the issue, so the “boffins” decided that 1 1:12″ twist would fix the problem, and it did.

Fast-forward to the 1980s and the development of the M-16A2. Those paying attention will recall being a bit mystified when discovering the twist rate was a radically faster 1:7″.

Now, the “new” ball round adopted was essentially a take on the FN developed SS109, and called the M-855. This ammo worked fine in the A2 but was woeful in the older, slower-twist A1 rifles.

NOTHING to do with the WEIGHT of the bullet (62gn nominal as opposed to the M-193 55gn pill). Because the SS109 / M855 has a composite construction, a longer boat-tail and a slightly longer ogive, it was LONG for its weight. US military team had been using the slightly-“dumpy” Sierra 63 gn bullet for “extended-range Match shooting for a couple of decades. That bullet, despite being about 12% heavier than the M-193 pill is about the same length and, as long as it is drive pretty hard, will stabilize nicely in a 1:12″ tube on a .223 / 5.56 rifle.

It gets more interesting. M-855 will stabilize in a 1:11″ twist barrel on a .223, so why did “the big kids” go with 1:7″?

TRACER ammo.

The same “big kids’ wanted a companion tracer bullet for both rifle and machine-gun use. Not just any old tracer, but one that would still be burning brightly out past 600 metres. This was a big “ask” in such a small-diameter bullet, so, they made the bullet LONGER, a LOT longer, to hold sufficient tracer compound to fit the bill. Just to make things even more interesting, the OVERALL length of the loaded round could NOT exceed the length of the ball round or it would not fit the rifle magazines or the feed mechs of the “MINIMI” / M-249.

In order to match the trajectory (sort-of) of the M-855, some creativity was applied to the propellant used, because of the amount of case space taken up by the long tail of the tracer bullet.

Ant that is why 1:7″ was chosen.

It got interesting over the next few years, with more “carbine’ variants appearing. I first noticed it in the late 1980s with the scoring differences between the 20″ and 16″ barreled F-88 (Oz Steyr AUG).

Troops with the carbines were getting consistently better groups (and scores)at 500m + than those with the “full-length” rifles, even though both systems had exactly the same 1″7″ twist-rate.

The carbine MV was lower, as one would expect, but because of this, the ROTATIONAL velocity of the bullet was also reduced. Still stable, however.ALL rifle bullets have to work with the factors of gravity and aerodynamic drag being against them. There is another factor, dynamic stability. Sir Isaac Newton had a few things to say about bodies in motion. A spinning bullet transitions from barrel to atmosphere on the “line of departure”, which is nominally line drawn from the last couple of inches of barrel at the time of launch. The spin imparted to the bullet at that instant is there to keep the bullet “flying true’. Now, that’s fine in a gravity-free vacuum. Back in the real world, gravity is trying to drag the bullet downwards and air-resistance is trying to slow its forward movement down. Not much is happening with rotational velocity.

If as we have all been taught, the bullet’s path is a nominal parabola, at some point, the bullet WILL be deviating from the line of departure. Thus, its physical alignment will diverge from that parabola. at that instant, supersonic aerodynamics take over and the bullet does a drunken twitch back to being coaxially aligned with the trajectory. Now, if these “adjustments are TINY and frequent, the disturbance to “precision” will be small. However, if the bullet is, effectively, OVER-STABILIZED, when the aerodynamic forces finally “bite”, the adjustment will be somewhat larger and the actual trajectory may become a bit “bent”. The further down-range that happens, the more disturbed the “precision” will become.

Extreme long-range shooters will have noticed “irregularities with “relative group size” at different ranges. A LOT of weird stuff happens in a very few seconds.