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Hi, Dave:
The only published BC I've seen for WFN bullets is Federal's, as discussed in this post, and I've looked.

Since Federal is rating their 300 .44 WFN at .215, I'd say .20 is a good guess for your 265 grainer. Right now I'm playing with the Lyman 358664 cowboy FN and using .15 as a guess.

You can be sure that every BTB bullet has a different BC. The BC equals sectional density over shape. Shape is called the form factor and is usually i in the equation, so BC or C = SD / i. So every time we change weight or diameter, we change SD, and if we change shape, say from a WFN to a LFN, we change i. To add to the fun, Sierra found that changing size can change i, even though the bullet is scaled up or down exactly. There are formulas for calculating the BC of typical high-power jacketed bullets, but the grease grooves in cast bullets throw in an extra curve. Lyman found that some pointed cast bullets had lower BCs than similar but blunter bullets. The theory is that the blunt point throws the shock wave away from the grease grooves, but the sharp point doesn't, and allows the grease grooves to create extra drag.

Most bullet manufacturers use the G1 drag function, and the few that don't say so. IIRC, Berger uses the G7 curve for his VLD bullets. This is sensible, as the G1 is a poor fit for VLD bullets that are used at a 1000 yards. Errors at that range are serious.

The BCs of the different drag functions are different, so your plain vanilla 30 calibre 180 grain spizter has a G1 BC of about .4, but it could be .2 or .8 in another drag function. Ingalls (GI) and G1 are close enough for most users to interchange. For more check out this short article.

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