Quicker twist rates have assumed great importance in improving the accuracy of modern rifles as well as shaping the cartridges we shoot today. They have played a major role in the development of the latest beltless super magnums and their ability to shoot long, heavy bullets accurately at longer ranges.
We live in an age of speed, when long-ranging overbore cartridges have become the flavour of the month. But to get those enhanced ultra-high velocities requires barrels to have a faster twist rate (that is, the speed of the spin imparted on the bullet by the spiral grooves cut into the bore) in order to stabilise bullets that seem to keep on growing longer and heavier every day.
Why the rate of twist is so important to accuracy? In simple terms, the barrel twist starts the bullet spinning at a given rate to stabilise the bullet. Generally, the twist rate should suit the heaviest (longest) bullet that might be used.
For benchrest and target work, the twist should precisely match the bullet. Over-stabilising a bullet has little negative effect on accuracy, except in the most extreme instances, but too slow a twist will definitely have a detrimental effect.
A lot hinges on the gyroscopic stability of the bullet. There are two basic rules for bullets:
- For a given size, a heavier mass is more resistant to disturbing forces than a light mass.
- The higher the rotational speed (ie, the faster the spin), the greater the rigidity, gyroscopic inertia and resistance to deflection.
Long 6.5mm bullets are inclined to a slight yaw, or wobble. Initial yaw as the bullet leaves the muzzle is governed mostly by its stability. This yaw prevents the bullet from striking straight-on, can affect grouping at 100m, and can last up to 200m before settling down. This accounts for some 6.5mm calibre rifles shooting just as good groups at the longer distance.
A constant rate of twist is important and today rifle manufacturers tend to err on the fast side, so that their guns will be capable of handling the new breed of long, heavy bullets designed for long-range hunting and competition shooting.
If you look at any of the current crop of custom .308 tactical or target rifles, you are almost certain to see the barrel has a twist rate in the 1:10 to 1:11.25 range rather than the standard 1:12. That’s because the 1:12 twist usually shoots lighter projectiles very well (eg, the Hornady 155gn BTHP Match and Sierra 168gn MatchKing) but that twist rate rarely shoots the more popular 175gn bullets well, let alone the 208gn BTHP Match.
Increasing the twist rate (but not radically) has no adverse effect on lighter bullets, and is necessary to ensure stability with heavier projectiles.
If you are a handloader, be aware of the velocity if your barrel barely meets the recommended twist rates. It is not unusual for a starting load to deliver indifferent accuracy, while the same bullet with a near-maximum load delivers tighter groups.
This is particularly true of magnums; the best accuracy often comes from near or maximum loads.
The new Nosler beltless magnums were developed to give the flattest possible trajectory and long-range punch for big game hunting, coupled with the best possible accuracy. Nosler has designed long, heavy bullets for this very purpose.
For example, take the 6.5 PRC, which has about the same powder capacity as the .270 Winchester and a faster 1:8 twist to handle ballistically more efficient projectiles.
Due to the smaller bore size, it doesn’t quite achieve .270 Win muzzle velocities in the same barrel length, but 6.5mm bullets like the Nosler 142gn ABLR have a higher BC (.719). Although they start out a smidgin slower, they catch up with typical .270 bullets in about 100m and are traveling faster beyond.
A BRIEF HISTORY OF SPIN
The rate of twist in a barrel assumed increased importance with the popularity of the .223 Remington factory loads with bullets weighing from 35 to 77 grains.
Shooters couldn’t figure out why their rifles shot so poorly with heavy bullets, until we woke up to fact that the standard twist of 1:12” was the culprit.
Rifles soon appeared which had faster twists and 1:9 became the norm for ARs, and even faster twists were employed by shooters who wanted to shoot 75 to 80gn projectiles from their .223s.
But long before the advent of the .223, twist rates raised heated discussion when the .243 Winchester and .244 Remington were introduced in 1955.
Although many handloaders preferred the shape of the .244’s case and slightly larger capacity, the .243 outsold it and the .244 become semi-obsolete. The.244’s lack of success was attributed to its 1:12 twist rate being too slow to stabilise the 100gn spitzer bullet that American deer hunters favoured.
It didn’t help, either, that Remington loaded the .244 with a 90gn bullet and touted it as varmint cartridge, whereas Winchester gave the .243 a 1:10 twist that stablised heavy 100 and 105gn bullets and called it a deer cartridge.
Remington didn’t do anything to rectify this until eight years later, when in 1963 they renamed the .244 as 6mm Remington and gave it a 1:9 twist. But they’d left their run too late. The .243 had become solidly entrenched and remains so right up to the present day.
Remington didn’t profit by their mistake, however, and committed the same blunder when they brought out the .260 Remington in 1966 by giving it a 1:10 twist which was too slow to stabilise the long, heavy 140gn 6.5mm bullets that had made the calibre’s reputation for deep penetration.
Remington eventually decided to change the .260’s twist rate to 1:9, the same as the earlier 6.5 Rem Mag, but at the round’s modest velocity this made it only a marginal performer with 140gn longsters. A 1:8 twist is more nearly ideal for the .260.
Consequently, this mild-shooting hunting and target cartridge failed to gain the popularity it richly deserved, being handicapped from birth by having too slow a twist rate.
Winchester made the same mistake when they lumbered their .264 with a 1:9 twist, but the round’s high velocity compensates somewhat.