Forums
New posts
Search forums
What's new
Articles
Latest reviews
Author list
Classifieds
Log in
Register
What's new
Search
Search
Search titles and first posts only
Search titles only
By:
New posts
Search forums
Menu
Log in
Register
Install the app
Install
Forums
Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Let's argue about BC's
JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser
.
Reply to thread
Message
<blockquote data-quote="Michael Courtney" data-source="post: 479315" data-attributes="member: 28191"><p>I apologize that I don't have time to speak to every question that has been raised, but I hope to summarize some of the science in an orderly way to promote a more scientific and less emotional discussion. There are several hypotheses related to why observations with different rifles might yield different estimates of ballistic coefficient. I tend to group these in two categories:</p><p> </p><p><strong>Rifle-centered differences:</strong></p><p> </p><p>Yaw/gyroscopic precession increases drag during the first part of the flight until it damps out and the bullet "goes to sleep." Rifles that tend to shoot bullets with greater peak yaw early in the trajectory will produce more drag, on average. These rifles probably also produce more shot-to-shot variations in drag. Two ideas mentioned as sources of larger yaw are imperfections in the crown and thin barrels. More data is necessary to determine how common this phenomena is and how much drag increase it creates, although Bryan Litz's computations are compelling that if the initial yaw angle is 11 degrees then the drag increase over the first 100 yards is near 7%.</p><p> </p><p>Deformation of the bullet in the barrel results in increased drag. Bullets are swaged down to the barrel diameter. This swaging leads to a smaller cross sectional area and higher sectional density in tighter bores, but it might also lead to deformations in the ogive and boat tail if too much metal needs somewhere to go. Different rifles also vary greatly in the depth and roughness of the rifling markes imprinted on the bullet. Finally, the extreme accelerations of some cartridges cause the lead in the nose to obturate and change the nose shape. Relatively small dimensional changes can produce significant differences in aerodynamic drag. This hypothesis is probably best tested by comparing bullets shot from sabots (thus no rifling marks) with bullets shot in the usual way. A careful experimental design will be needed to minimize possible confounding factors. </p><p> </p><p><strong>Bullet-centered differences:</strong></p><p> </p><p>Tests have shown 1-4% variations in bullet BCs even within the same box of bullets. Most of the bullet-to-bullet variations within the same box are thought to be related to inconsistencies in the bullet tip, as these can be reduced in plastic tipped bullets and by using post-manufacture dies to ensure greater tip uniformity. Some useful references are:</p><p> </p><p><a href="http://www.6mmbr.com/bulletpointer.html" target="_blank">Whidden Bullet Pointing Die System</a></p><p> </p><p><a href="http://www.appliedballisticsllc.com/index_files/Berger155grainVLD.pdf" target="_blank">http://www.appliedballisticsllc.com/index_files/Berger155grainVLD.pdf</a></p><p> </p><p>Bullets can show more pronounced dimensional variations between lots, and this can produce significant BC variations. In a few cases the manufacturer will announce the switch to a new die along with a difference in their published BC. In other cases, these dimensional variations are not announced by the manufacturer. We've measured average BC variations as large as 13% between different lots. (Our BC numbers in the earlier post showing rifle-to-rifle variations are from the same lot of 125 Nosler Ballistic Tips, so we attribute the measured differences to the rifles). </p><p> </p><p>Michael Courtney</p></blockquote><p></p>
[QUOTE="Michael Courtney, post: 479315, member: 28191"] I apologize that I don't have time to speak to every question that has been raised, but I hope to summarize some of the science in an orderly way to promote a more scientific and less emotional discussion. There are several hypotheses related to why observations with different rifles might yield different estimates of ballistic coefficient. I tend to group these in two categories: [B]Rifle-centered differences:[/B] Yaw/gyroscopic precession increases drag during the first part of the flight until it damps out and the bullet "goes to sleep." Rifles that tend to shoot bullets with greater peak yaw early in the trajectory will produce more drag, on average. These rifles probably also produce more shot-to-shot variations in drag. Two ideas mentioned as sources of larger yaw are imperfections in the crown and thin barrels. More data is necessary to determine how common this phenomena is and how much drag increase it creates, although Bryan Litz's computations are compelling that if the initial yaw angle is 11 degrees then the drag increase over the first 100 yards is near 7%. Deformation of the bullet in the barrel results in increased drag. Bullets are swaged down to the barrel diameter. This swaging leads to a smaller cross sectional area and higher sectional density in tighter bores, but it might also lead to deformations in the ogive and boat tail if too much metal needs somewhere to go. Different rifles also vary greatly in the depth and roughness of the rifling markes imprinted on the bullet. Finally, the extreme accelerations of some cartridges cause the lead in the nose to obturate and change the nose shape. Relatively small dimensional changes can produce significant differences in aerodynamic drag. This hypothesis is probably best tested by comparing bullets shot from sabots (thus no rifling marks) with bullets shot in the usual way. A careful experimental design will be needed to minimize possible confounding factors. [B]Bullet-centered differences:[/B] Tests have shown 1-4% variations in bullet BCs even within the same box of bullets. Most of the bullet-to-bullet variations within the same box are thought to be related to inconsistencies in the bullet tip, as these can be reduced in plastic tipped bullets and by using post-manufacture dies to ensure greater tip uniformity. Some useful references are: [url=http://www.6mmbr.com/bulletpointer.html]Whidden Bullet Pointing Die System[/url] [URL]http://www.appliedballisticsllc.com/index_files/Berger155grainVLD.pdf[/URL] Bullets can show more pronounced dimensional variations between lots, and this can produce significant BC variations. In a few cases the manufacturer will announce the switch to a new die along with a difference in their published BC. In other cases, these dimensional variations are not announced by the manufacturer. We've measured average BC variations as large as 13% between different lots. (Our BC numbers in the earlier post showing rifle-to-rifle variations are from the same lot of 125 Nosler Ballistic Tips, so we attribute the measured differences to the rifles). Michael Courtney [/QUOTE]
Insert quotes…
Verification
Post reply
Forums
Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Let's argue about BC's
Top