A recently published magazine article* did an experiment to verify the relationship between barrel rifling twist rate and accuracy with different bullet weights. Unintentionally, the data generated reaffirmed one of the main reasons for reloading. Optimizing a bullet/reloading recipe for a specific rifle follows closely behind cost as a major incentive to reload. The authors used the .223 Remington for the experiment because of the large difference in bullet weights available and the easy availability of a variety of barrel twist rates. Bullet weights ranged from 35 grains to 90 grains and twist rates used were 1:7, 1:8, 1:9, and 1:12. The firearms used went from 16” barreled carbines to 26” heavy barreled rifles.
To be fair, there are other variables besides bullet weight that determine optimum twist rate. The proliferation of longer all copper or copper alloy bullets has demonstrated that longer bullets require a faster twist to be adequately stabilized for a given bullet weight.
The experiment clearly proved that the slow 1:12 twist rate did not stabilize the heavier 77 grain and 90 grain bullets. Group size was over 5.5” for the 77 grain bullet and over 26” for the 90 grain bullet. It is also interesting that the 1:12 twist barrel shot the 35 grain bullets into a 1.4” group and the heavier 45 grain bullets into a tighter 0.6” group. The two 1:7 twist rifles shot almost all of the bullets into less than 2” groups except one rifle did not like the 45 grain bullet. It shot that bullet into a 2.3”group. Those are not terrible results for a carbine, but not as good as the rest of the bullets.
Two 1:7 twist rate rifles were used. One was a 16” carbine and the other a 26” heavy barrel rifle. The longer barrel shot all of the bullet weights better than the shorter barrel except the 69 grain bullet. The shorter barrel shot that bullet measurably better. I am not sure if barrel length or barrel quality is the driving force here. Since I have seen a pistol out-shoot an accurate 30.06 rifle I own at 100 yards, I am inclined to discount barrel length.
It would be nice if you could look at the data and be able to say that the optimum twist for the .223 Remington is X. The 1:9 twist 26” heavy varmint barrel shot all of the bullets 77 grain and below at 1” or less. That is impressive. This same barrel shot the 90 grain bullets into a 6.5” group, clearly unstabilized. One barrel does not make a serious study. All it proves is this is an exceptional barrel. Without much more data, I would not pronounce the 1:9 twist barrel optimum for all but the heaviest .223 bullets. I would wish that I personally owned a rifle with a barrel this accurate and versatile.
Clearly, barrels show a preference for different bullet/load recipes. The non reloader can purchase several different boxes of cartridges with bullets near the recommended weight for caliber and barrel twist rate. The best available cartridge may still not be the best that rifle can shoot. The handloader can optimize a load recipe for each of those bullets, selecting the most accurate of all as the pet load for that rifle. Solving problems like this is what makes reloading such a great hobby!
*Shooting Times, April 2012, 175 Groups Fired! Twist Rates & Accuracy, p. 50, authors R.L. Window & Dick Metcalf