January 04, 2011
For many years, rate of twist was a topic left to gunsmiths and benchrest competitors.
But with the increased popularity of the .223 Remington came factory loads with bullets weighing from 36 to 77 grains, and many shooters had no idea why their rifles shot so well with some loads and so poorly with others. Some figured it out pretty quickly. Eventually, faster twist rates like 1:9 became the norm for ARs, and even faster twist rates became more popular with shooters keen on shooting 75- to 77-grain projectiles from their .223s. Today, twist rate is a common topic of discussion at the shooting range. But twist rates played a major role in shaping the guns we shoot today long before they were a common topic of conversation. The 6mm Remington is one example that comes to mind.
Though the 6mm Remington's larger case capacity endeared it to handloaders, the .243 Winchester kicked its tail. The lackluster success of the 6mm Rem. can be directly attributed to its 1:12 twist rate that would not stabilize the 100-grain bullets deer hunters coveted. Heck, it didn't even do a good job of stabilizing Remington's 90-grain factory load.
Winchester's .243, however, had a 1:10 rate of twist, and it did a marvelous job of stabilizing 100-grain deer bullets as well as lighter varmint bullets. By the time Remington changed to a 1:9 twist rate, the damage was done. The .243 Win. dominated the 6mm market and still does to this day.
The .260 Remington is another victim of a bungled twist rate. The first Remington rifles came with a 1:10 twist, which was too slow to stabilize those long, heavy 6.5mm bullets. Remington switched to a 1:9 twist rate, but it was still too slow and provided iffy accuracy at best with 140-grain bullets. A 1:8 twist rate is a much better choice for the .260. Consequently, this phenomenal hunting and target round has languished in obscurity until recently because it was crippled from the start with the wrong twist rate.
So, you might ask, what does the barrel twist do and why is it so important?
The short answer is that barrel twist starts the bullet spinning at a given rate to stabilize the bullet. The rate must be constant for the utmost accuracy. An increasing twist rate will have little effect on accuracy except in the most extreme cases. However, a decreasing twist rate will almost certainly result in a dramatic decrease in accuracy.
The explanation for this is simple. According to barrel maker Dan Lilja, "If we look at a recovered bullet, we'll notice that the rifling cuts a partial helix in the bearing surface of the bullet for each land. The helix is on an angle matched by the twist in the barrel. If the twist rate decreased, the angle of this helix would decrease also and would effectively cut a wider groove into the bullet. This condition is undesirable because the bullet could then yaw while still inside the barrel. It would lack the full support of the barrel, especially on the driving side of the land. The yaw would be demonstrated by poor grouping on a target once it was released from the barrel.
"Conversely, an increase in rifling pitch would tighten the angle. While this is no better than a constant twist on the bullet, it is certainly better than a decrease in twist."
If we accept that a constant rate of twist is important, we have to determine the proper rate. Fortunately, a quick call to any gunsmith or barrel maker worth his shop apron will uncover the right answer for any of the common calibers and bullet weights. But determining the proper rate back in the day required a bit of math and the Greenhill Formula.
The Greenhill Formula for most standard cartridges is T=150(d/r), where T is the twist rate, d is the bullet diameter, and r is the bullet length to diameter ratio (bullet length divided by its diameter). For cartridges with a muzzle velocity of more than 2,800 fps, substitute 180 for 150.
If we were to do the Greenhill Formula on Sierra's .308 168-grain MatchKing, we would come up with an ideal twist rate of 1:11.76. That's close enough to 1:12, which is a common twist rate for .308 and one that does very well with 168-grain bullets.
The numbers on the 175-grain MatchKing, which has a diameter ratio of 4.081 (1.257 inches divided by .308), look like this: T=150 x .308/4.081, which gives us an optimum twist rate of 1:11.32.
That 0.44-inch difference may not seem like a big deal, but it is. As an example, take a look at just about any custom .308 tactical rifle these days, and you are almost certain to see a barrel with a twist rate in the 1:10 to 1:11.25 range. That's because the classic 1:12 twist usually shoots lighter projectiles--155-grain Palma bullets and 168-grain MatchKings--just fine, but that twist rate rarely shoots the more popular 175-grain bullets well at all. Speeding up the twist a bit has no adverse effect on lighter bullets, but it ensures stability with heavier projectiles. And that's a plus in anyone's book.
If you're a handloader, take note of velocities if your barrel barely meets the recommended twist rates. I've seen starting loads shoot fair to terrible, while the same bullet over a bit more powder shot the lights out. Clearly, in that situation, the rate of twist was just right; it will get the job done, but a slightly faster twist wouldn't hurt.
Over-stabilization as a result of a too-fast twist rate can occur, resulting in a bullet that travels along its downward arc with its tip pointing skyward, exacerbating wind drift and hastening velocity loss. Ideally, the tip of a properly stabilized bullet should tip downward as the bullet begins its downward arc. Clearly, an over-stabilized bullet is not conducive to accuracy, but it is only noticeable in extreme cases of over-stabilization and at very long range. You'd never notice it at hunting distances.
It is easy to get swept up in trying to figure out the "perfect" twist rate. But the fact of the matter is that getting it down to the nth degree, at least for the casual shooter, is largely a theoretical exercise. If you're contemplating the rate of twist for your next custom build, I suggest you save yourself the aggravation and ask your gunsmith what he suggests for the bullets you plan to shoot. That way, you end up with a rifle that shoots a variety of loads well, rather than a fickle rig you'll give up on.