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GUNS Magazine March 2013 Digital Edition - Page 12
tigHt turns rifling twist rates Have been cHanging due to longer and longer bullets. im Carmichel, whose wisdom I trust in all things involving the rifle, once wrote, “No shooting subject is more likely to make one sound like an expert, and at the same time prove him a fool, than a discussion of rifling twist.” And since Jim went on to say, “I hope I am quoted on this,” I’m happy to do so, and in turn, hope I can avoid sounding foolish. Twist is generally measured in terms of distance traveled for one revolution of the projectile, for example one turn in ten inches, usually written as 1:10". Twist can also be expressed in terms of calibers (e.g. a “40-caliber twist,” or in the angle of the rifling grooves relative to the bore axis, but we’ll stick with rotation/distance terms. The purpose of rifling, of course, is to impart spin to the projectile so it follows a more consistent and predictable path. A projectile with little or no spin tends to go straight for a short distance, then move about unpredictably—which is why in baseball, when a knuckleballer is on the mound pitching, the catcher wears an oversize catcher’s mitt. A sphere is relatively easy to stabilize, so early muzzle-loading rifles had twists such as 1:66". The longer a projectile is, the faster it must be spinning to stabilize. Since heavier bullets are usually longer we tend to equate spin with bullet weight, though in fact bullet length is what matters. Bullet stability depends on how rapidly the bullet is spinning. Rate of spin depends on two factors: rifling twist, and bullet velocity. Compare two 0.224" barrels, both with a 1:12" twist, both firing identical 50-grain bullets. One is J dave anderson Longer bullets, such as monometal and VLD styles (above), need to be spinning faster in order to stabilize. For instance, these 7mm bullets are long for caliber except the heavy traditional roundnose. These 7mms include (from left to right) the Hornady 175-grain roundnose, Hornady 154-grain SST, Barnes 150-grain TTSX and Berger 168-grain VLD. Rifling twists have gotten faster over the years because spitzer bullets are longer than roundnose bullets (below); monometal bullets are less dense than leadcore bullets and are longer for the same weight such as these examples (from left) 0.224" 55-grain Speer FMJ and Hornady Spire Point, 0.284" 120-grain Nosler and Barnes X, 0.308" 200-grain Speer roundnose, 180 Hornady Spire Point, 165 Barnes TSX. This Ruger 77 Hawkeye in .223 Rem has a 1:9" twist and is very accurate with every load Dave tried, from 40 to 75 grains. If Dave wanted to use longer 80-grain bullets or some of the sleek VLD bullets he’d likely need a 1:8" twist. Scope is a Leupold Mk 4 3.5-10X. chambered in .222 Rem, with 3,000 fps muzzle velocity. At one turn per foot, and going 3,000 fps, it exits the bore spinning at 3,000 revolutions per second, 180,000 rpm. The other is a .220 Swift with muzzle velocity of 4,000 fps. The bullet exits spinning at 4,000 revolutions per second, or 240,000 rpm. If we want the same rate of spin from our .222 Rem, we’ll need a barrel with a 1:9" twist, which at a muzzle velocity of 3,000 fps spins the bullet at 240,000 rpm. If a bullet isn’t spinning fast enough the result is obvious; bullets yaw until they lose all semblance of stability and accuracy, tumble end for end, and hit the target flying sideways if they hit it at all. The negative effects on a bullet spinning faster than necessary are less obvious; in fact there may not be any. If the bullet is out of balance, spinning it faster just makes things worse, though with today’s bullets you have to search hard to find a bad one. A thin-jacketed bullet spun too fast may simply fly apart in flight. Now there’s a fairly obvious effect, though one a shooter can easily 12 W W W. G U N S M AG A Z I N E . C O M • M A R C H 2 0 1 3