# ??? for JBM.

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Quarterbore, Mar 11, 2004.

1. ### QuarterboreWell-Known Member

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103
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Jan 31, 2003
I bought some Sierra 155 palma MK's Supposed to have a B.C. of .450 w/ velocity over 2600f/s. Question, is that G1 function BC,and if so what is the G5 funtion B.C.?

Thanks, I will be testing them this weekend along with the 155 gr.Nosler J-4s with same BC.

2. ### JBMWell-Known Member

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113
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Jan 12, 2004
If it's published by Sierra, it's going to be a G1. I've never seen them publish anything else.

On my website in the downloads section there is a file that is a reprint of some drag functions from "Exterior Ballistics of Small Arms Projectiles". At 2600 f/s, you can see that the G1 drag function is 0.2959 and 0.1869. Since G1/BC1 = G5/BC5 (the CD of the bullet is the same, so these terms have to be the same for the same atmospheric conditions) so BC5 = BC1 * G5/G1 = 0.450 * (0.1869/0.2959) = 0.284.

Now if you run two trajectories, one with the G1 BC and the other with the G5 BC you can see they are pretty close (it convinces me that I didn't mess up the math). They aren't exactly the same though because we used only one BC at one velocity.

3. ### QuarterboreWell-Known Member

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103
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Jan 31, 2003
Thanks Alot!!!