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GB, how bought some imfo on interpeting chrono results related to load development

1322 Views 4 Replies 4 Participants Last post by  .45 COLT
How important are the chrono numbers to you?

Are you more concerned with SD or ES?

Do you have any insight on the juggling of powder and primer combo's to jock the numbers?

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Let me begin by saying that I prefer the lowest possible velocity spread. That's probably just an engineer's hangup, seeking repeatable results.

Extreme spread (ES) can be accentuated with minor glitches that are soon overcome. Standard deviation (SD), to my way of thinking, is more important. Put another way, suppose that you fire five shots at the following velocities: 3008, 2997, 3017, 3001 and 2550. The ES looks really bad, but the SD more nearly reflects that four of the five velocities were good.

To me one of the most important chronograph uses is to measure the velocity loss over a known distance, for example 100 yards. The velocity loss over the known distance allows you to determine the real ballistic coefficient (BC). If you know the already BC of the bullet, then you gain an insight into the bullet's stability. More yaw equals more drag. The yaw may be exaggerated by a high muzzle exit pressure, a twist problem, etc., but you know it's there thanks to the chrono.

If a bullet has a factory stated BC of .300 but your chrono leads you to conclude that the BC is .273, then you have learned valuable information. The ultimate advantage of finding the real BC is that you will be able to better predict the trajectory and windage of your bullet.

My knowledge of primer/powder combo's is more focused around research cannon. I don't feel that I can responsibly say off the top of my head, "Use 29 grains of XYZ powder with a Winchester magnum primer." Internal ballistics as it relates to pressure issues is a lot less predictable than most engineers would have you believe. Loads commercially sold are carefully developed by ammunition makers. Despite the advances in software, I am told that over 100,000 rounds of the new .17 rimfires were fired to prove their performance and safety.
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