Many rifle shooters remain semistuck in the British mists of 1879, when a math professor named George Greenhill developed his famous formula for calculating rifling twist-rates. His formula is the reason many riflemen ask, "What's the heaviest bullet XYZ twist will stabilize?" Today this is like asking, "What's Queen Victoria up to?"
While Greenhill's formula included "inputs" for both bullet weight and length, in 1879 almost all bullets were blunt-nosed and made entirely of lead alloy, so bullet weight correlated with length. Even after smokeless powder and jacketed bullets appeared in the 1880s, bullets were still about the same. Even most 21st Century shooters still believe bullet weight is the factor in rifling twist.
Instead, bullet length is the biggie. Today, bullets of the same weight vary considerably in length, including many made entirely of copper or brass, often with miracle-polymer tips. Rifling-twist formulas have changed since 1879 because of that, and are particularly helpful when handloading the .223 Remington. Those .223 barrels have twists almost as variable as economic theory.
The .223 appeared in 1964 as the commercial version of the 5.56 NATO military round, with a standard rifling twist of 1 turn in 12" (written 1:12), slightly faster than the 1:14 then common among .22 centerfires. This was because the twist in military barrels had just been increased to 1:12 to fully stabilize the 55-gr. service bullet. However, some manufacturers put 1:14 barrels on .223s because they were already cranking 'em out for other cartridges.
Back then most shooters believed rifling twists should be barely enough to stabilize a given bullet, due to the poor balance of many jacketed bullets. Poorly balanced bullets resemble fake news--inaccurate in the first place, and with added spin even less accurate. Today's bullets are very well balanced, so they'll shoot accurately even when "over stabilized."
The twist in military 5.56 NATO barrels eventually increased to 1:7 to stabilize heavier bullets, starting a fast-twist trend for the .223. Unfortunately, there's no strict definition of "fast twist." Most factory ,223s now have twists of 1:9 or 1:8, but some have 1:7 twists, and a few remain stuck on 1:12.
Handloaders need to know the rifling twist of their .223s, otherwise their chosen bullet might fly sideways. Some manufacturers stamp the twist rate on .223 barrels, while others don't even reveal it on their website. If...