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Ballistic Coefficients Tested

 
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  #1  
Old 07-06-2002, 06:13 PM
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Ballistic Coefficients Tested

Forgive me if this is old news, but I just found the April Shooting Times that I never had a chance to read while cleaning house recently.

In it there's an article by Rick Jamison on Ballistic Coefficients. The article is OK, but what's really interesting is the test data. He shot about 50 common 30 caliber "hunting" bullets over his Oehler--damn I need to get one of those things!--(10 shot strings of each bullet) and measured their BC's. The results are pretty interesting. The absolute numbers are lower than the advertised BC's but I'm guessing that's because he tested during cold weather and didn't correct them to standard conditions (like the advertised numbers supposedly are).

Anyway, the relative numbers (how the different bullets compare with each other) look like solid data.

Some observations:

Pointy tipped bullets rock. In the weights that they are made the Scirocco, SST and Ballistic Tips kicked serious butt on everything else. That may come as no surprize to some but it may be to others. It's amazing how many people will try to tell you that meplat size doesn't matter much--it's all the ogive, etc. If you plan on shooting at long range a semi-spitzer, protected point type shape DOES make a difference for the worse.

The BT's and the Sciroccos generally advertise very similar BC's and the SST's advertise lower. Between those three bullets, the SST and Scirocco ran neck and neck beating the Balistic Tips by quite a bit. It seems either Hornady is being conservative with their ratings or Nosler is bragging a bit. But who knows what methods they use to test these things anyway....

Speaking of bragging (or telling tall tales), I've suspected that Barnes overrates the BC's for their X bullets for quite a while. They just don't add up. This test seems to confirm it. For example, the 180 XBT is rated at .552 while the Scirocco is only rated at .505 (.047 less). In the test, the Scirocco beats the XBT by .034, a difference of .081. That's quite a discrepancy. The 200 X (flat base with big hollow point) has an advertised BC of .55, nearly the same as Sierra's Soft Point Boat Tail. But the Sierra beats the X by .071. That's enough of a difference to make a hit become a miss at long range if somebody takes the advertised numbers at face value.

***DISCLAIMER***

I wasn't there to supervise the testing. BC's vary depending upon muzzle velocity, twist rate and even change in flight as they lose velocity (sometimes getting better, sometimes getting worse), etc. A single number is really only a starting point, something to use for comparisons sake...yada, yada, yada....

***END DISCLAIMER***

Anyway, I thought this would be an interesting topic for discussion. It may not be all-inclusive, but that's the best BC test data I've seen published in a gun rag yet. Better than nothing, I guess.

It also reinforces the notion that if you want to shoot at an animal at long range while hunting, you damn well better have practiced at that range with that load already. Your computer generated chart just might be wrong (at no fault of the computer or the software)!
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Old 07-14-2002, 05:39 PM
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Re: Ballistic Coefficients Tested

Jon...

I didn't read that artical (I stopped reading Shooting Times about a hundred years ago [img]images/icons/wink.gif[/img] ).

You didn't say which Oehler he used, but I'm assuming it was the M43 shooting lab.

The assigning of a BC varies from maker to maker... some (like Sierra) bend over backward to get the best scientific data available, and they were the first to have software to take into account changing BC errors that occured with their softwear (they use G1 BC's for all bullet shapes, even 9 degree BT's).

Other companies just look at the bullet and guess, and the guesses are usually optimistic (no big surprise!!)... but some are also too low.

The reason for publishing BC's (other than advertising) is to be able to predict bullet placement with programs. But most of the BC programs use different math formulas to achieve the end results. The result of this, is that the BC's are different for the same bullet, using different software... if you run a 175 grain, 30 cal, Matchking on the Sierra program, and get the drop figures, and then run the same bullet, with "Brand X" software, you can get a completely different set of numbers. [img]images/icons/frown.gif[/img] [img]images/icons/frown.gif[/img]

The problem is how do you get the BC's for the software you are using... with Sierra bullets, that's easy... their BC's are adjusted, to run properly with their software... but with some of the others, it's trial and error.

I recently started using Hornady 58 gr V-Max bullets in a 40-XBBR 6mmBR, Crow and Egg rifle. Hornady says the BC is .250.
But @ .250, the bullets weren't tracking.

I ran groups at 100, 200, 300, and 400 yards, and took the actual measurments, and then started trying different BC's until I found which form factor it matched, and the exact trace I measured... it turned out to be a G1, with a .305 BC (Aberdeen protocol)... much higher than they advertise.

I think that "we" (very long range shooters) push the envelope a lot harder that most shooters. The 1000 match shooters don't need this information to be super accurate, cuz they get sighters, but when you are ranging targets out way past where the average guy shoots, and need DAMN GOOD DATA, then you may have to juggle the numbers, and generate your own data.

CatShooter.
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Old 07-14-2002, 10:22 PM
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Re: Ballistic Coefficients Tested

Good points, CatShooter. I've played with programs that use different curves, will correct BC's for use on different curves, etc and you do get different results. That's why I believe that the computer is a great tool for getting you in the ballpark, but test results are needed at long range.

The point I was trying to make is that most manufacturers say their BC's are based on the G1 curve, corrected for standard conditions. So that should give you a fairly accurate way to compare bullet to bullet (but definately not perfect). But when a bullet is WAY off, you can use any curve you want and it will still be way off.
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Old 07-15-2002, 04:27 AM
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Re: Ballistic Coefficients Tested

I've chosen Barnes for the high BC in the past combining that with a tough bullet. After buying the Oehler 43 PBL and acoustic target and testing many bullets now the verdict is in with Barnes. A big thunbs down I'm affraid. My 416 400gr are nowhere near what they claim. I already knew my old crony was screwy or the BC was. Turned out it was a big.... Both of them! My 400's were going 2650 not 2550 and the BC was WAY lower than advertised. I'll post some results later when I get home tomarrow. The scirocco's were almost exactly what was advertized with every weight of thirty cal I tested. Although they almost never grouped well in my guns, save one.

His findings are in Aprils issue, maybe I missed that when I himmed and hawwed over renewing this spring. I'm a sucker, I finally did.
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