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GUNS Magazine April 2010 - Page 8

HANDLOADING • John Barsness • SPINNING ANEw The twists of rifling twist. piral rifling was developed in what are now the countries of S Austria and Germany in the late 15th and early 16th centuries, but didn’t catch on all that fast, primarily because of the difficulty Don published an article about my 4-to-1 formula (April 2009 issue), for figuring the potential velocity changes due to case capacity. Don played with the actual physics of the problem, just because he wanted to know why such a simple formula worked, and published the results in Varmint Hunter magazine. Don’s math is far over my head (I was a biology major, not an engineer), but it was nice to know somebody thought enough of the 4-to-1 rule to find out why it worked. Don also became intrigued with rifling twist, and eventually published an article with some new formulas in Precision Shooting. Don’s formulas have become accepted in certain circles; in fact Bryan Litz, an aeronautics engineer and shooter who recently published the fine book Applied Ballistics For Long Range Shooting, cites Don’s formulas, including those for variations in muzzle velocity and atmospheric pressure. These formulas are published in Applied Ballistics—but more importantly (at least for most of us) they’re incorporated into the ballistics of fitting the projectile to the bore, especially if it was full of blackpowder fouling. Still, by the late 1700s rifles were overtaking smoothbores as the firearm of choice in some parts of the world, and over the next century the invention of conical bullets, selfcontained metallic cartridges and smokeless powder made rifles truly practical. One of the results was the invention of the first formula for figuring the correct rifling twist for projectiles. This was done empirically for centuries, primarily because bullets (and cannonballs) were all round, so they didn’t require much twist. In 1879, however, a British mathematics professor, Sir Alfred George Greenhill, developed a formula for lead-cored conical bullets some people still use. The big problem is that in 1879 muzzle velocities were much lower than they would soon be, with the advent of smokeless powder, and muzzle velocity also has an effect on bullet stabilization. A modern modification of the Greenhill formula substitutes 180 instead of 150 for bullets started at higher velocities, but the fact is modifying the Greenhill formula is kind of like dressing up a really old pig. Newer formulas are more precise. My friend Don Miller, a retired engineer and enthusiastic shooter, likes to spend time fiddling with various aspects of ballistics. Among other things, a couple of years ago In The Beginning When expressed mathematically the Greenhill formula appears more complex than it is. In practice it’s really simple: (1) Measure the length of a bullet in inches, then divide by the bullet diameter. (2) Divide 150 by the result of (1). (3) Multiply the result of (2) times bullet diameter. This product is the supposedly ideal rifling twist for that bullet, expressed in inches. As an example, let’s figure out the rifling twist for a typical 180-grain 30-caliber spitzer boattail, in this instance the 200-grain Nosler Partition, 1.38" long: (1) 1.38 ÷ .308 = 4.48 (2) 150 ÷ 4.48 = 33.48 (3) 33.48 x .308 = 10.31 So according to Greenhill, rifling of about 1 turn in 10" is needed to stabilize a 200-grain Nosler Partition. This is remarkably close to the 1:10" twist used in most .30 caliber centerfires, but this doesn’t mean the Greenhill formula is perfect, or even all that applicable today. 8 “Over-stabilizing” of bullets isn’t as big a problem as it used to be, thanks to better balanced bullets.ThisSIG556DMRshot50-grainNoslerBallisticTipsverywell,despiteariflingtwistthat’s theoreticallytoofast. WWW.GUNSMAGAZINE.COM • APRIL 2010

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