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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Does velocity affect ballistic coefficient?
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<blockquote data-quote="CONatureBoy" data-source="post: 2275509" data-attributes="member: 118769"><p>Nobody has commented yet about MagnetoSpeed chronograph accuracy:</p><p></p><p>[ATTACH=full]294988[/ATTACH]</p><p></p><p>So at velocities around 3,000 fps, your chronograph could be as much as 15 fps lower or higher than the reading. At 70 degrees F, sea level, 2" high scope, 200-yard zero, 0.284 bullet diameter, 145 grains, and a G7 BC of 0.230, the Berger calculator says the difference will be about a half-inch of vertical at 500 yards. That's not your three inches, but it's something. Depending on your ballistics calculator's features, a variety of other minor environmental factors (e.g. the <a href="https://en.wikipedia.org/wiki/External_ballistics#Magnus_effect" target="_blank">Magnus effect</a>, whereby a crosswind out beyond 200 yards could pull your shot upwards) might contribute another inch or two that your calculator doesn't account for. Litz's book (cited above) spells them out. You might want to figure out how much of the three inches that surprised you came from the environment, and how much from the gun and load. I would also be careful (1) not to hastily attribute all three inches to a single effect, and (2) to distinguish adjustments for muzzle velocity from adjustments to BC assumptions. In his <em>Modern Advancements in Long Range Shooting</em>, Vol. II, Litz says (p. 227), "A concept that many long-range shooters are familiar with is truing or calibrating the ballistic solver. . . . This process is necessary . . . where shooters may not have good information on their bullet's muzzle velocity or BC. . . . It is possible to determine MV with reasonable accuracy based on observed drop. However shooting to determine BC or drag is a very different thing which is much more difficult to do accurately." If you want precise (+/- 1%), empirically determined, velocity-dependent BCs for a given bullet, you'll usually find them in Litz's book, <em>Ballistic Performance of Rifle Bullets</em>, 3rd Ed.</p></blockquote><p></p>
[QUOTE="CONatureBoy, post: 2275509, member: 118769"] Nobody has commented yet about MagnetoSpeed chronograph accuracy: [ATTACH type="full"]294988[/ATTACH] So at velocities around 3,000 fps, your chronograph could be as much as 15 fps lower or higher than the reading. At 70 degrees F, sea level, 2" high scope, 200-yard zero, 0.284 bullet diameter, 145 grains, and a G7 BC of 0.230, the Berger calculator says the difference will be about a half-inch of vertical at 500 yards. That's not your three inches, but it's something. Depending on your ballistics calculator's features, a variety of other minor environmental factors (e.g. the [URL='https://en.wikipedia.org/wiki/External_ballistics#Magnus_effect']Magnus effect[/URL], whereby a crosswind out beyond 200 yards could pull your shot upwards) might contribute another inch or two that your calculator doesn't account for. Litz's book (cited above) spells them out. You might want to figure out how much of the three inches that surprised you came from the environment, and how much from the gun and load. I would also be careful (1) not to hastily attribute all three inches to a single effect, and (2) to distinguish adjustments for muzzle velocity from adjustments to BC assumptions. In his [I]Modern Advancements in Long Range Shooting[/I], Vol. II, Litz says (p. 227), "A concept that many long-range shooters are familiar with is truing or calibrating the ballistic solver. . . . This process is necessary . . . where shooters may not have good information on their bullet's muzzle velocity or BC. . . . It is possible to determine MV with reasonable accuracy based on observed drop. However shooting to determine BC or drag is a very different thing which is much more difficult to do accurately." If you want precise (+/- 1%), empirically determined, velocity-dependent BCs for a given bullet, you'll usually find them in Litz's book, [I]Ballistic Performance of Rifle Bullets[/I], 3rd Ed. [/QUOTE]
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