Bryan Litz with, once again, some science that makes our hair hurt, and that’s even in his cut-down version with minimal traumatic math. In a post excerpted from his latest book, Modern Advancements in Long Range Shooting, Litz discusses the results of tests with a rifle and a series of barrels manufactured by the same barrelsmith to differ only in twist rate, and be identical in all other aspects — a controlled experiment.
Through our testing, we’ve learned that adequate spin-stabilization is important to achieving the best BC (and lowest drag). In other words, if you don’t spin your bullets fast enough (with sufficient twist rate), the BC of your bullets may be less than optimal. That means, in practical terms, that your bullets drop more quickly and deflect more in the wind (other factors being equal). Spin your bullets faster, and you can optimize your BC for best performance.
Any test that’s designed to study BC effects has to be carefully controlled in the sense that the variables are isolated. To this end, barrels were ordered from a single barrel smith, chambered and headspaced to the same rifle, with the only difference being the twist rate of the barrels. In this test, 3 pairs of barrels were used. In .224 caliber, 1:9” and 1:7” twist. In .243 caliber it was 1:10” and 1:8”, and in .30 caliber it was 1:12” and 1:10”. Other than the twist rates, each pair of barrels was identical in length, contour, and had similar round counts.
There’s quite a lot to get your skull around here, and even when you Read The Whole Thing™ (which you’re totally gonna do, right?) there’s stuff that’s hard to understand.
We wonder what the mechanism is that, in effect, raises the drag (and BC) of an underspun bullet, and what we think it is is a form of precession. Instead of spinning perfectly around its longitudinal axis, the bullet wobbles a little bit off axis. Instead of going along a perfect line, and therefore staying in a single point, as viewed in 2D from dead ahead, the point of the bullet is describing, when reduced to two dimensions, a small circle… in three dimensions, the point is spiraling towards the target even as the bullet’s center of mass is proceeding directly targetwards. There are several ways that this could raise the drag of a typically supersonic bullet. One is simply that the off-axis bullet may present a larger frontal area (or larger average frontal area, if the precessing bullet has a changing frontal area) to the slipstream. Another is that flow might separate irregularly from the tail of the bullet. Turbulent, separated flow induces buckets of drag. There are probably others that we don’t get because, unlike Bryan Litz, we’re not aerodynamicists by training.
One thing that Litz points out is that you may be getting very satisfactory groups, and still not optimum BC. Why does that matter? If your groups are OK and your BC is suboptimum, who cares? Well, BC (as Litz shows, practically a function of gyroscopic stability) also influences accurate range, for example.
It’s a common assumption that if a shooter is seeing great groups and round holes, that he’s seeing the full potential BC of the bullets. These tests did not support that assumption. It’s quite common to shoot very tight groups and have round bullet holes while your BC is compromised by as much as 10% or more. This is probably the most practical and important take-away from this test.
Like all of Litz’s research, this is some fascinating stuff. The same series of tests also showed that twist rate affects muzzle velocity, but very little. It’s intuitive that a higher twist rate would, by imparting more friction to the projectile, decrease muzzle velocity. The results, though, showed that while twist rate affects MV a statistically significant amount, that amount is extremely low. As Litz puts it, himself, in a couple of the comments to the post:
The scatter in the data and the R squared value indicate that only about 1/2 the variation in MV is due to twist rate (Correlation Coefficient is 0.55) which means that random noise has as much effect as twist rate. This is discussed further in the book, as well as similar results presented for a different bullet in which the relationship was even weaker, and the correlation was lower.
Remember that the correlation quotient between twist rate and BC was 0.87. Random chance probability is 0.50, so unlike the twist-to-BC correlation, the twist-to-MV correlation is weak as water… but it’s still there. It’s a measure of Bryan Litz’s painstaking care in collecting this data that the 0.55 correlation even shows up in the data table, but it does, as a low double-digit variation in MV with each change in twist rate!
The point in presenting these results is to show that the effect of twist rate on MV is VERY minor, and can almost be said to be statistically “in the noise”. ….
The long and short of it is that regardless of caliber and bullet weight, twist rate has very little effect on MV. You’ll see more fps difference per inch of twist on a .220 Swift just because you’re dealing with higher velocities. In other words, the percentage of MV change due to twist is pretty consistent.
That’s an example of how the comments are as good as a second, followup post in terms of their educational value. If you had asked us, we’d have said that, so long as the bullet was stabilized at some minimum level, twist rate would have had a minimal effect on accuracy, but a larger one on MV. And yet Bryan Litz’s results are exactly the opposite of what we’d have said on instinct. Obviously we didn’t understand this as well as we thought!
So read the post and comments — and keep reading till you understand it all, which may take those of us who are reformed infantrymen more than one reading. And if you want a deeper dive in the physics of accuracy, Modern Advancements in Long Range Shooting, and Litz’s other books are available from Applied Ballistics directly or from Amazom.com (at a glance, it looks like you save money by going to Applied Ballistics).