HANDLOADING • JOHN BARSNESS • Ballistic Coefficient There’s only way to find the bottom of this mystery. M any handloaders have a touching faith in the listed ballistic coefficient (BC) of their rifle bullets. In fact many choose bullets based on this number, something at least partly the fault of gunwriters, especially many who wrote in the 1960s, when many writers “discovered” technical stuff such as ballistic coefficient and kinetic energy. As an innocent kid, I read lots of articles claiming BC and energy were the truly important things in hunting bullets. It was some time before I gained enough experience to realize many of the writers making that claim hardly ever hunted. They were right about one thing, however: A high BC allows a bullet to shoot flatter and resist wind-drift better, though exactly how much a shooter should worry about these things depends on the shooting involved. BC means just about zilch to a woods hunter, no matter what rifle he carries, but it means a lot more to an open-country hunter or longer-range target shooter. Before World War II discussions of BC were pretty rare in shooting essays, but after the war a whole generation became more technically oriented. Handloading manuals even started publishing the BC’s of bullets, though in a rather crude way. I own a copy of the first Speer handloading manual, purchased at a garage sale in Lewistown, Montana. The BC’s listed only go two decimal places. Their 180-grain 30-caliber spitzer, for instance, is listed at .49, a level of imprecision considered inadequate by today’s sophisticated bullet junkies. Actually .49 was probably a guess anyway. For many years listed BC’s of rifle bullets were arrived at by comparing the bullet to a “form factor,” not actual shooting. Other factors were also often involved in such guesses. I have heard through the rifle-loony grapevine that at least one bullet company listed BCs far lower than reality for their boattailed bullets, primarily because the company president hated boattails. At the same time another company, known for their boattailed bullets, listed higher BCs than actual shooting indicated. Similarly, after Ballistic coefficient doesn’t matter at 80 yards, the range of the shot on this Texas whitetail, yet some woods hunters still obsess over it. plastic-tipped boattails became popular, one company made a 7mm bullet with a very high listed BC. A year or two later a competitor brought out a plastic-tipped 7mm of the same weight with a listed BC .001 higher than the first company’s. The truth is BC isn’t a firm number, like 140 grains of bullet weight. The same bullet’s BC varies according to different factors, including velocity. If you go to the Sierra Bullets Web site you’ll find a listing of BCs for all their bullets at different velocity levels. For instance, their 150-grain roundnose .308" bullet, meant primarily for the .30-30 Winchester, has a BC of .200 at velocities over 2,700 fps. The BC increases to .227 between 1,700 and 2,700 fps, and to .270 under 1,700 fps. Their 155-grain .308" hollowpoint boattail MatchKing, however, has a listed BC of .417 at velocities over 2,800 fps. This drops to .397 at 1,800 to 2,800 fps, and to .355 at under 1,800 fps. So the effects of velocity on BC are directly opposite in these two bullets. Standard Conditions Only In any such reference, the BC listed is for what’s known as Army Standard Metro Environment conditions: sea level, 59 degrees Fahrenheit, 78 percent humidity and a barometric pressure of 29.53. Vary any of those conditions and BC varies as well. Elevation has the biggest effect. The BC of, say, a typical 55-grain plastic-tipped 22-caliber varmint bullet can increase from around .250 at sea level to well over .300 in the Rocky Mountains. I first discovered this myself in reverse many years ago on a trip to West Virginia where a couple of days were spent shooting “ground hogs” (woodchucks) at ranges up to 600 yards. I’d been shooting western rockchucks, another wild marmot, for many years and knew from experience how much bullets drifted in the windier parts of the Wild West. Well, the same bullets drifted a lot more in comparatively mild breezes in the lower, thicker air of West Virginia. Temperature follows elevation in its effect on BC. Colder air is thicker, so 16 WWW.GUNSMAGAZINE.COM • NOVEMBER <strong>2009</strong>
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