G1 vs. G7 ballistic coefficient

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trazman
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G1 vs. G7 ballistic coefficient

Post by trazman » Mon Feb 02, 2015 7:18 pm

What is the ballistic coefficient?

Basically the ballistic coefficient(BC) is a measure of how well a bullet penetrates the air. It depends on bullet weight, cross-sectional area and form factor. BC is measured in lb/in².

Which is better G1 or G7?
The simple answer to that is the G1 value normally works better for shorter flat-based bullets, while the G7 value should work better for longer, boat-tailed bullets. Usually G1 BC are higher than G7 which doesn't mean we should use the higher one but that one that is more suitable for our bullet. We can find out this by simply look at the bullets we are using. If the bullet is a flat base bullet similar to fig.1 we use G1 BC if the bullet is a boat tail similar to the fig.2 we use G7 BC.
g1g7shape.png
Ok now we know that a G7 BC is better for calculating drop for boat tail bullets but we still don't know why. A G1 BC is only good for certain velocities and changes when velocity drops. The G7 standard bullet form factor is more similar to a boat tail bullet and so it is almost identical at all velocities. The second reason why we want to use G7 BCs instead of G1 BC is that usually we dont know at which velocity the G1 BC was measured. Usually bullet makers tend to market higher G1 BCs that are only suitable for high velocities, because shooters want as high BC as possible. Sierra for example publish more than one G1 BC for the bullet:

Example 1:
Sierra Match King HPBT 190gr code 2210
G1 ballistic coeficients
.533 at 2100fps and above
.525 between 2100 and 1600fps
.515 below 1600fps

This data is good when we have a ballistic software that does not offer a G7 BC input.

Bryan Litz has published in his book Applied Ballistics for Long-Range Shooting an average G1 BC of .523 for SMK from example 1 with a variation of .069. The G7 BC for the same bullet is 0.268 with the variation of only .005.

What have we learned:

1.G7 is not better then G1 but it is only better with bullets with boat tails that are better for long range shooting. A G1 BC is better fo short flat based bullets.

2.We want to use a G7 BC in a ballistic calculator if possible or we have to insert multiple BCs
as seen in example 1 if the G7 is not supported by our calculator.

3.Even that a G1 BC has a higher number than a G7 Bc we cannot compare them, but we want to use the G7 BC for our long range boat-tailed bullets and we want the highest G7 BC possible.

If something is not clear I am more than happy to explain it further just tell me what you don't understand.
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Tomaž Ražman
European Long Range Hunting
http://eu-lrh.com

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