Re: Long bullets require fast twist barrels?
You're seeing that in a general sense it's better to have enough twist and greater, than not enough twist. A more detailed review reveals 'best' twist(Sg) for each bullet, load, and shooting condition.
Best, or best balance of all pros/cons, can also be tied to particular use.
Folklore spins out of misunderstandings, and actually end up pseudo facts with funny mutations.
Bullets going to sleep is an example of a minor factor in external ballistics that has been hogwashed to explain all kinds of unrelated things.
When someone declares that overspinning bullets is bad for accuracy, it really means overspinning is not quite as good as perfect spin. And of course at worst overspinning leads to jacket failures.
But it's usually easier to see underspinning as a 'problem'.
Also, predicting the perfect speed is not as easy (with some bullets) as running numbers through a rule of thumb calculation.. It still comes down to center of pressure to center of gravity length that disturbing moments leverage, and these can't always be measured with calipers. A resulting example here being that ideal Sg for flat base BR bullets(~1.3) is lower than ideal Sg for VLDs(~1.5), depending on the bias of your calculation method. With this, point blank BR shooters do go lower in twist than prudent for longer range BR shooters.
But if you shoot short flat base bullets with a twist producing Sg of ~1.5, or even ~1.8, they will still shoot fine, as will long VLDs. Drop both down to calculated Sg of ~1.2 and it's likely both will shoot worse. With high muzzle pressures(barrel too short) and any less than a perfect crown, an Sg of ~1.1 may not be enough to prevent tumbling. And at ~1.0 bullets will immediately tumble.
This is why it's easier to see underspin than overspin.
I picked up a Cooper in 223rem, they come in 14tw. After buying, it dawned on me that 14tw is stupid, as the standard for this round is 12tw. I was right, there are few over the counter bullets that would work well out of 14tw.
I did some calcs and tried 50Blitzkings with an Sg just under 1.2 here at sea level. Best I could get was ~1/2moa from them. Then I found 50Jayners(FB BR bullet) with a calculated Sg of 1.3, and my grouping dropped immediately to ~1/4moa. It was like flipping a switch.
Where I hunt with these bullets(upper PA) their Sg is 1.44, and I have a very high kill percentage with them inside 600yds.
So I'm left to wonder, if the Cooper were in 12tw, would I have reached this point accuracy-wise?
I would have gone to 55-60gr bullets, better in the wind, but ya never know about accuracy. The gun would have bucked more, the powders would be slower, and my barrel of the same length would produce higher muzzle pressures.
See, there is a lot to it that isn't in rules of thumb.