Considering the importance of shot-to-shot Ballistic Coefficient (BC) consistency, it’s important to understand the different ways in which variations can arise. One way you can get shot-to-shot variation in BC is due to the stability of the bullet. Even within this narrow topic, there are several avenues to explore.
If the twist rate in your rifle barrel is not fast enough to stabilize the bullet right out of the muzzle (meaning the SG is less than 1.5), there are a couple consequences.
- The average BC of the bullets will be lowered.
- The consistency of the BC will also be compromised.
The above consequences are somewhat obvious and we see them easily in the Doppler radar tracks. But most shooters don’t have this problem because twist rates and bullets are selected such that bullets are usually launched with SG’s over 1.5.
The other avenue where stability can play a role in BC/drag variation is when the bullets approach transonic speed. Even if you fire a bullet with adequate stability from the muzzle (SG is 1.5 or higher), the bullet can still experience stability issues at transonic speed, which is around 1340 fps depending on temperature. The figure below shows an example of this for a 6.5mm 135 grain bullet. The bullet flies with good consistency from the muzzle down to about Mach 1.3 (high transonic), then the shot-to-shot drag begins to fan out.
This issue of transonic instability and BC variation occurs when a bullet design is not properly designed, or intended for use at transonic range.
When Berger is developing a bullet for use at long range, the designs are deliberately made to achieve transonic stability, and radar testing is conducted at long range to insure favorable, consistent performance. If it’s not, we go back to the drawing board and revise the design. Sometimes this results in a lower average BC, but it’s a consistent BC from shot-to-shot, which is more important for keeping groups small at long range, and hitting targets.
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What is a BC?
BC, Accuracy, and Precision
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