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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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  #1  
Old 10-25-2011, 9:07 AM
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jimmykan jimmykan is offline
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Default Ballistic Coefficients: G1 vs G7

I just wanted to share an explanation of why some bullet manufacturers publish multiple ballistic coefficients of the same bullet for different velocity ranges.

Here's what Brian Litz of Berger Bullets has to say:

Quote:
The reason why the BC of a modern long range bullet changes so much at different velocities is because modern bullets are so different in shape compared to the G1 standard that its BC is based on. In other words, the drag of a modern long range bullet changes differently than the G1 standard projectile, so the coefficient relating the two (the ballistic coefficient) has to change with velocity.
The standard G1 projectile profile:


A typical bullet that closely matches the G1 profile and needs only one G1 BC to describe its drag characteristics across a wide velocity range:


The standard G7 projectile profile:


A typical bullet that closely matches the G7 profile and needs only one G7 BC to describe its drag characteristics across a wide velocity range:


Quote:
As you can see, the G7 standard projectile, with its long boat tail and pointed ogive bears a much stronger resemblance to a modern long range bullet than the G1 standard projectile. As a result, the BC of a modern long range bullet thatís referenced to the G7 standard is constant for all velocities! In other words, a trajectory thatís calculated with a ĎG7 BCí doesnít suffer from the same velocity dependence problems and inaccuracies as calculations that are made with a G1 BC.
In other words when you calculate bullet flight path using a drag model that does not closely match the shape of the bullet, you need to compensate by using different BCs for different velocity ranges.

When you choose the right drag model, one BC value describes the bullet's flight path from start to finish.
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Old 10-25-2011, 11:16 AM
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Great in-a-nutshell summation Jimmy!

Folks, when shooting long range, one should use the BC that fits their bullet's profile. For nearly all of the long range projectiles that means using the G7 for calculations.

For example:

Lets look at a .308 launching the 175 SMK at 2500 FPS.

Using G1 BC: AT 1000 yards we get about 41.9 MOA or 438.4 inches of drop with a retained velocity of about 1187 FPS.

Using the G7 BC (more accurate data): At 1000 yards we get 43.3 MOA or 453 inches of drop with a retained velocity of about 1098. That is subsonic at sea level.


The difference is important. If you were basing your data on the G1 you would think you are fine for 1000 yards; however, if you based your data on the G7 BC, you would know that 2500 fps is a little too slow for 1000 yards at sea level by only a few FPS.

Yes, it is that important!

If you are wondering why many manufactures publish only the G1 BC, it is because the G1 is a larger number and posting the G7 (always a lower number) will, most likely, confuse the prospective buyer who does not understand the difference.
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Old 10-25-2011, 10:58 PM
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Quote:
Originally Posted by jimmykan View Post
In other words when you calculate bullet flight path using a drag model that does not closely match the shape of the bullet, you need to compensate by using different BCs for different velocity ranges.

When you choose the right drag model, one BC value describes the bullet's flight path from start to finish.
well i guess that answered my question i had earlier. funny thing is that i know i've read this article off appliedballisticsllc or somewhere off the jbm site, but i have never caught that part... and on to more reading to see what else i have missed and can learn
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