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Old May 28, 1999, 12:30 PM   #1
Little-e
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I was introduced to reloading a while back and learned handgun only. Now I'm teaching rifle loading to myself and have really gotten into it. I'm at the point where I have a chronograph to help track my results but ballistic coefficient is driving me crazy. What is it in simple terms? How is it helpful and how do you figure it out? I've read about it in several mags. but it just is not sinking into my thick noggin.
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Old May 29, 1999, 11:24 AM   #2
Futo Inu
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Basically, it's the SHAPE and DENSITY of the bullet. Most bullets are of fairly similar density, so the basis for comparison among bullets usually turns on their SHAPE, if I'm not mistaken. The BC is a measure of the bullet's ability to "cut through the air" with minimal drag; thus the higher the BC, the less bullet "drop" at the same distance out with the same muzzle velocity. So a bullet with higher BC has about the same drop at long distances as a bullet with a little more velocity but less BC coefficient. Basically, certain ratios of length to width make for better BCs regardless of the absolute size of the bullet. For example, 7mm (.284) bullets have the best BC in the 145-165 grain range, but .308 bullets have other widths beat in the 170-200 grain range, and the .338s have the best BC with about 250 grains. And, in addition to the general width to length ratio, certain specific bullet charactistics increase BC, though a little less important than general length to width ratio, I think. For example, the sharper the point of the bullet nose, the higher the BC (just makes sense, a point cuts through air better than a round nose). Also, boat tails have a slight edge over flat base. and the actual rate of change of the curves from the flat sides of the bullet near the base as it goes toward the point is important too. Though I'm not sure what kind of curve is best - I'm thinking slowly rounded is better than distinct angle. So, clear as mud?
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Old May 29, 1999, 11:27 AM   #3
Futo Inu
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Oh, I should add that, from looking at ballistics tables, it appears to me that BC is far, far LESS important than muzzle vel in determining bullet drop/point blank range, at least at the vast majority of normal hunting ranges (out to 300-350 yards).
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Old June 3, 1999, 12:03 AM   #4
Cheapo
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The shape also has an effect on BC, sometimes quite large.

For example, the .30-06 M2 Ball bullet is nominally 150 grains, and goes subsonic (thus getting whipped about by wind a lot) somewhere between 700 an 800 yards, IIRC. But the 7.62 NATO, by adding a boattail and using a secant ogive (specific front-end shape that looks less aerodynamic than the gentle curve of the M2), gets at least an extra hundred yards, with measurably less wind deflection before the dreaded subsonic zone. I think that's even with a MV of 75 fps less...don't have those military data sheets handy...

And it weighs 147 grains, nominal. Okay, it's only 1/30th less, but there is a difference.

The VLD (very low drag) design of the newer Sierra 155-gr Palma Matchking makes it carry downrange as well as design of the revered 1962 or whatever year it was introduced 168-gr Matchking. But since it can be launched faster, the 155-gr carries all the way out to 1,000 yards from a .308. The 168 needs a .30-06 or better to be used at 1,000 yds.

The decimal value of the BC is supposed to be how well the actual velocity loss compares to some mathematical "ideal" shape. It's a ratio of less than perfect, in my understanding. Larger decimals are closer to 1, which is "perfect" but impossible in air.
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Old June 9, 1999, 03:47 AM   #5
Unkel Gilbey
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The Special Ball ammo that we used at 1000 yard matches with the M14 featured the old 173 gn Spire point, boat-tailed bullet. We preferred the "Brown box" Special Ball to the "White box" match ammo at a 1000 because of it's slightly higher muzzle velocity. As I understand it, the 30-06 match ammo also used the 173 gn bullet, and it was the ammo of choice for Carlos Hathcock when he went out hunting down in SE Asia... Have never seen any info on the BC of this 173 gn bullet, but given that it was in the 170-200 gn range, I'd hazard a guess that it was a fairly efficient bullet in those respects. My .02 worth!
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Old June 9, 1999, 08:48 PM   #6
Art Eatman
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The Sierra loading manual has an excellent discussion of Ballastic Coefficient.
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Old June 29, 1999, 12:43 AM   #7
headroom
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The BC on the M118 special ball is .530, according to the army.
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Old June 29, 1999, 08:30 AM   #8
bfoster
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Note that small differences in B.C. (say on the order of 0.02) may not give an advanatge in actual use. If you ever set out to measure this, you will find that not every shot gives the same result. It is entirely possible for a bullet of slightly lower rated B.C. to outperform one of slightly higher rated B.C. due to consistency- not just of the bullets, but of how the rifle handles the entire load. I'd also recommend actually checking where the rifle you are loading for actually shoots at longer ranges. At least one major manufacturer of component bullets appears to consistently take figures from the high end of their test results for what they publish. A final point: I've yet to see a manufacturer publish seperate data for coated bullets. In some instances, moly coated bullets show a slightly higher or lower B.C. than uncoated bullets.
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Old July 1, 1999, 10:03 AM   #9
flatlander
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Several years ago, some of us tried to use the Sierra 168gr.MK's in our M1A's at 100 yds. We found that, because of the shape of the bullet, and our rifles' chambers, the bullet wouldn't settle down, or "go to sleep" at 100yds. It was still a little wobbly and didn't group well until out to at least 200yds. Now moly coating is supposed to let the bullet slip a little and align itself better with the bore, so that as it exits the muzzle, there's not so much yaw or wobble to damp out. This lets the bullet "go to sleep" sooner, and results in better retained velocity, or in effect, higher effective BC. This is all pretty much theory, but it does explain the demonstrated higher BC's some experimenters have been getting with moly bullets, using two chronographs, one at the muzzle, the other downrange. The actual BC can be computed using the two velocity readings, and it's quite common for the actual or "realtime" BC to change along with atmospheric conditions.

[This message has been edited by flatlander (edited July 01, 1999).]
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