Home Forums Hunting & Shooting Ask The Gunwriters Why the BC difference?

In .284 caliber, the 156g Norma Oryx has a listed BC of 0.330 on the MidwayUSA site. In comparison, the 160g Woodleigh bullet has a listed BC of 0.486. They both appear to be protected points bullet and of similar shape, so I was wondering if the listed BC of those bullets are that dissimilar.

Anyone shoot either/both?

How does field data relate to listed BCs?

Thanks in advance.

Kyreloader, for a given form and caliber BC is proportional to weight.

Just looked at the 160g Speer Deep Curl which looks like a similar bullet. (I understand the bonding is different). The listed BC for these bullets in 0.455.

Is the shape of the Oryx that much different?

I wouldn't know to be truthful, never looked at any of them. It doesn't take much difference in shape to cause a shift in BC, and for that matter the manufacturer's BC is an approximation which will vary from one shot to the next and due to changes in atmospheric pressure/temperature and/or velocity due to changes in charge weight, bore quality, Mondays thru Fridays and the weekend is a crap shoot too. Simple eh?

Rough rule of thumb(s):

About 70% of bullet drag results from the nose form and most of that as a result of meplat width. About 5% comes from parasitic drag on the bullet shank and the balance is a matter of bullet base design, ie flat base, boat tail. All of the above is variable. For a given caliber, velocity and bullet weight the difference in drop between spitzer and round nose forms does not become large until passing 300 +/- yards.

End of the day, if you're hunting big game at reasonable distances it's not something to lose sleep over. Take a look at this, there will be a 100 question test in 30 minutes.

http://www.exteriorballistics.com/ebexplained/index.cfm

Ballistic coefficients are often calculated rather than actually measured by firing. Many manufacturers tend to exaggerate BC, especially if a rival company offers a similar bullet.

A number of years ago one company introduced a certain plastic-tip bullet with a BC of, let's say, .456. Not longer after another company offered a plastic-tip in the same diameter and weight with a listed BC of .457.

Just from looking at the two side by side the woodleigh has less of a tangent ogive, and a smaller meplat, and the little extra weight doesn't hurt either.

A couple of times several years ago, some of us gun witers cornered guys from the bullet-manufacturers and asked 'em how they got their ballistic coefficients. They answered the same, virtually word for word �

"We calculate 'em as close as we can, then we see what our competition claims for theirs."

For that very reason, when I compared my .220 Howell and .224 Howell to a .220 Swift factory load, I used Hornady data for their Swift factory ammo, their 50-grain V-Max, and their 75-grain A-Max.

I figure that any proprietary fudge factor that Hornady may have used for their ballistic coefficients should presumably be the same (thus legitimately comparable) for both bullets.

I honestly don't pay much attention to BC. My shots aren't long enough for it to matter that much.

The bullets I shoot tend to have good BC's but that's because I shoot a little heavier spire point bullet (which is usually longer) in most of the calibers I use.

How a bullet performs on bone and tough shot angles over my velocity range is what I use to judge 'em.

YMMV.

JM