Understanding the relationship between bullets and barrel twist helps prevent mistakes. Here’s what you need to know…
Why am I devoting this space this time to such a topic? Well, because it’s commonly asked about, and, no doubt, because it influences some of the decisions and options faced in choosing the best-performing load for our needs. Making a mistake in choosing twist can limit both the selection and performance in the range of usable bullet weights and styles.
First, barrel twist rate is a component in the architecture of the barrel lands and grooves. The lands and grooves form a spiral, a twist, that imparts spin to a bullet, and the rate of twist is expressed in terms of how far in inches a bullet travels to make one full rotation. “1-10” (one-in-ten) for example means “one full rotation for each ten inches of travel.”
Bullet length, not weight, determines how much rotation is necessary for stability. Twist rate suggestions, though, are most usually given with respect to bullet weight, but that’s more of a generality for convenience’s sake, I think. The reason is that with the introduction of higher-ballistic-coefficient bullet designs, which are longer than conventional forms, it is easily possible to have two same-weight bullets that won’t both stabilize from the same twist rate.
The M-16/AR15 barrel changes give a good example. Short history of mil-spec twist rates: Originally it was a 1-12, which was pretty standard for .224-caliber varminting-type rounds, like .222 Remington, which were near-universally running bullet weights either 52- or 55-grain. That worked with the 55-grain FMJ ammo issued then. Later came the SS109 63-grain round, with a bullet that was a bit much for a 1-12. The military solution was total overkill: 1-7. That’s a very fast twist.
Commercially, the 1-9 twist became the standard for .223 Remington for years. It’s still popular, but is being replaced, as far as I can tell, by the 1-8. An increasingly wider selection of barrels are done up in this twist rate. I approve.
I’d always rather have a twist too fast than not fast enough. For a .223 Rem. 1-9 is not fast enough for anything longer than a routine 68-70-grain “magazine bullet,” like a Sierra 69gr MatchKing. 1-8 will stabilize any of the newer heavier bullets intended for magazine-box cartridge overall lengths, like a Sierra 77gr MatchKing. An 8 twist will also shoot most of the longer, higher-BC profiles, like the Sierra 80gr MatchKing (which is not intended to be assembled into a round that’s loaded down into a magazine).
Other popular calibers have likewise edged toward faster and faster “standard” twist rates, and that includes 6mm and .308. Once those were commonly found as 1-10 and 1-12, respectively, but now there’s more 1-7s and 1-9s offered. Reason is predictable: longer and heavier bullets, and mostly longer, have likewise become more commonly used in chamberings like .308 Winchester and 6XC.
The tell-tale for an unstable (wobbling or tumbling) bullet is an oblong hole in the target paper, a “keyhole,” and that means the bullet contacted the target at some attitude other than nose-first.
Base your next barrel twist rate decision on the longest, heaviest bullets you choose to use, and at the same time realize that the rate chosen has limited those choices. If the longest, heaviest bullet you’ll shoot (ever) is a 55-grain .224, then there’s honestly no reason not to use a 1-12. Likewise true for .308-caliber: unless you’re going over 200-grain bullet weight, a 1-10 will perform perfectly well. A rate that is a good deal too fast to suit a particular bullet may cause damage to that bullet (core/jacket integrity issues), and I have seen that happen with very light .224 bullets, like 45-grain, fired through, say, a 1-7 twist. At the least, with that great a mismatch you might not get the velocity up where it could be.
Bullet speed and barrel length have an influence on bullet stability, and a higher muzzle velocity through a longer tube will bring on more effect from the twist, but it’s a little too edgy if a particular bullet stabilizes only when running maximum velocity. My failed 90-grain .224 experiment is a good example of that: I could get them asleep in a 1-7 twist 25-inch barrel, which was chambered in .22 PPC, but could not get them stablized in a 20-inch 1-7 .223 Rem. The answer always is to get a twist that’s correct.
Effects on the load itself? Yes, a little at least. There is a tad amount more pressure from a faster-twist barrel using the same load, and the reason is initial bullet acceleration is slower.
The preceding was adapted from Glen’s newest book, Top-Grade Ammo, available here at Midsouth. For more information on this book, and others, plus articles and information for download, visit ZedikerPublishing.com