Faster twist rates are of great importance in improving the accuracy of modern rifles, as well as in the design of the cartridges we shoot today. They played a key role in the development of the latest beltless Super Magnums and their ability to accurately fire long, heavy bullets at longer ranges.
We live in an age of speed where long range overbore cartridges have become the flavor of the month. But to achieve these improved ultra-high velocities, barrels must have a faster twist rate (i.e., the rate of spin imparted to the bullet by the spiral grooves cut into the bore) to stabilize bullets, which seem to continue to grow longer and longer by the day heavier.
Why is twist rate so important for accuracy? Simply put, the rotation of the barrel causes the bullet to spin at a certain rate to stabilize the bullet. In general, the spin rate should match the heaviest (longest) bullet that could be used.
For benchrest and target work, the rotation should match the bullet exactly. Over-stabilizing a bullet has little negative impact on accuracy in all but the most extreme cases, but spinning too slowly is definitely detrimental.
A lot depends 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 perturbing forces than a light mass.
- The higher the rotational speed (i.e. the faster the spin), the greater the stiffness, gyroscopic inertia and resistance to deflection.
Long 6.5mm bullets tend to yaw or wobble slightly. Initial yaw as the bullet exits the muzzle is primarily determined by its stability. This yaw prevents the bullet from making a direct impact, can affect grouping at 100m, and can take up to 200m before it settles. This explains why some 6.5mm caliber rifles shoot groups just as well at longer ranges.
A consistent twist rate is important, and today’s rifle manufacturers tend to err on the fast side in order for their guns to be able to handle the new generation of long, heavy bullets being developed for long-range hunting and competitive shooting.
If you look at any of the current .308 custom tactical or target rifles, you will almost certainly find that the barrel has a twist rate in the range of 1:10 to 1:11.25 rather than the standard 1:12 velocity. This is because the 1:12 twist usually shoots lighter projectiles very well (e.g. the Hornady 155 gn BTHP Match and the Sierra 168 gn MatchKing), but this twist rate rarely shoots the more popular 175 gn bullets well , let alone the 208 gn BTHP match.
Increasing the twist rate (but not radically) has no adverse effects on lighter bullets and is necessary to ensure stability on heavier projectiles.
If you’re a handloader, watch out for speed if your barrel barely hits recommended twist rates. It is not uncommon for a starting charge to deliver uneven accuracy while the same bullet with a near maximum charge will deliver tighter groups.
This is especially true for magnums; The best accuracy often comes from near or maximum loads.
The new Nosler Beltless Magnums are designed to offer the flattest possible trajectory and long shot for big game hunting, coupled with the best possible accuracy. Nosler designed long and heavy bullets for just this purpose.
Take the 6.5 PRC, for example, which has roughly the same powder capacity as the .270 Winchester and a faster 1:8 twist to handle more ballistically efficient projectiles.
It doesn’t quite achieve .270 Win muzzle velocities for the same barrel length due to the smaller bore size, but 6.5mm bullets like the Nosler 142gn ABLR have a higher BC (.719). Although they start out a bit slower, they will catch typical .270 rounds in about 100m and fly faster beyond that.
A BRIEF HISTORY OF SPIN
Twist rate in a barrel gained importance with the popularity of .223 Remington factory loads with bullets ranging in weight from 35 to 77 grains.
The shooters couldn’t figure out why their heavy-duty bullet rifles fired so poorly until we woke up and realized that the standard 1:12 inch twist was the culprit.
Rifles with faster rotations soon appeared and 1:9 became the norm for ARs, and even faster rotations were used by shooters wanting to fire 75 to 80 gn rounds from their .223s.
But long before the advent of the .223, twist rates sparked heated debates when the .243 Winchester and .244 Remington were introduced in 1955.
Although many handloaders preferred the .244’s case shape and slightly larger capacity, the .243 outsold it and the .244 became semi-obsolete. The .244’s lack of success has been attributed to its 1:12 twist rate being too slow to stabilize the 100 gn spitzer bullet that American deer hunters preferred.
It also didn’t help that Remington loaded the .244 with a 90 gn ball and touted it as a varmint cartridge, while Winchester gave the .243 a 1:10 twist that stabilized heavy 100 and 105 gn bullets , and called her a deer cartridge.
Remington did nothing to correct this until eight years later in 1963 they renamed the .244 the 6mm Remington and gave it a 1:9 twist. But they had left their run too late. The .243 was firmly established and has remained so to this day.
Remington didn’t capitalize on their mistake, however, and made the same mistake when they released the .260 Remington in 1966, giving it a 1:10 twist that was too slow to accommodate the long, heavy 140gn 6.5 -mm bullets established the caliber’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 modest velocity of the round, this made them only a marginal performer with 140 gn Longsters. A 1:8 twist is nearly ideal for the .260.
As a result, this mild-firing hunting and targeting cartridge did not gain the popularity it deserved, being hampered from birth by too slow a twist rate.
Winchester made the same mistake when they rumbled their .264 with a 1:9 twist, but the cartridge’s high velocity makes up for that somewhat.