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Ballistic Coefficient (BC)

 
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  #1  
Old 11-01-2006, 07:31 AM
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Ballistic Coefficient (BC)

The Nosler Reloading Guide defines BC as "The ratio of a bullet's sectional density to its coefficient of form, used to describe the bullet's effectiveness in overcoming air resistance during flight."

Sectional Density - "The ratio of a bullet's weight, in pounds, to the square of its diameter, in inches."

Coefficient of form - Not defined.

If I understand BC correctly, a higher the BC:
1. Allows a truer flight path as the bullet passes through air with less resistance.
2. Allows a flatter trajectory as velocity is increased due to less resistance.
3. Allows the bullet to impact with increased energy due to increased velocity.

What I don't understand is how factors like materials and design are calculated into BC. How do metals like lead, copper and steel effect BC? How does a hollow point(SMK) or a delrin tip(Barnes X) effect BC? How does lead placement with in the bullet (some have lead throughout some have lead in the base) effect BC?

Where do bullet makers list the equations or test results that determine BC?

Rogue
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  #2  
Old 11-01-2006, 09:22 AM
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Re: Ballistic Coefficient (BC)

I'll do my best here. The lead placement is focused around balance and terminial ballistics. For example take a partition. The partition is placed where the bullet will balance with the lead on each side. The BC benifet is minute however the balance will help the overall aerodynamics which is to say, if a bullet is balanced it will overcome forces better or rather maintain itself. The rifling stabilizes the bullet by spinning the bullet if the bullet is balanced the bullet maintains better. A good example of what I am trying to say(rather poorley) is shooting a 80 grain .224 bullet out of a 1:14 twist barrel, the result is unbalanced due to a to slow of rate of twist, the bullet doesn't spin properley "as it was designed" therefore doesn't ballance.
I'm sorry that is the best I can explain the lead/placement thing.

As for the hollow points and plastic tips here is my shot at it. BC, simply put is how well does the bullet shead drag as you earilier stated. The plastic tip and the soft point closes any void and allows a point on the bullet to more effeciantly "cut the air" so to speak this reduces drag. So why is the hollow point so darn accurate with that drag producing hole in the front? How this works has alot to do with relative wind speed, supersonic aerodynamics and this is a subject that could keep me writing all day , so I will spell it out as simply as I can. The HP creates its own "airpoint" the air entering the hollow point becomes stagnet,and builds pressure, the pressure extend forward compressing the air ahead of the bullets path. The lower pressure around the bullet flows around the bullet as if it had a sharp point on it. This "airpoint does two main things It creates a sharper point but more importantly stabilizes the nose of the bullet. You may have seen that some people uniform the meplat on hollow points , If the meplat is not uniform the bullet will not create a centered "airpoint" so to speak. I know this is not explained real good but it is the best I can do with a computer right know. Hopefully this helps a little if it doesn't reply on what your questions are and I will try again.


RH [img]/ubbthreads/images/graemlins/confused.gif[/img]
If you haven't read in the hunting section there is a quiet instrusting issue that comes up in the post titled sheep hunting rifle. I have written a little in that post about BC
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  #3  
Old 11-01-2006, 09:43 PM
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Re: Ballistic Coefficient (BC)

Rotorhead - Thanks for the input!

I spent several hours today educating myself on what Balistic Coefficient is and how it is measured. I talked with bullet techs at Barnes, Nosler and Sierra. The Sierra 5th edition reloading book has a section in the back of the book titled "Exterior Ballistics." This section provides a wealth of information related to BC.

As there are current discussions concering BC, I offer the following:
The BC of any bullet is a constant until the bullet is fired. The moment the bullet touches the lands the BC becomes a variable. Altitude, temperature, barometeric pressure, relative humidity evelation angle, velocity, barrel quality and twist rate all effect a BC.

The Sierra Infinity Program can be used to calculate BC as the bullet is in flight. This procedure is explained in the Exterior Ballistics section of the book.

Rogue
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Old 11-02-2006, 05:26 AM
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Re: Ballistic Coefficient (BC)

[ QUOTE ]
So why is the hollow point so darn accurate with that drag producing hole in the front?

[/ QUOTE ]According to Sierra Bullets, hollow point bullets have a more uniform dimension at the junction of the body to the boattail/base. If one checks the bullets for out of round at this point using a V block and dial indicator reading in .0001-inch increments, the difference is obvious.

Sierra used to make their 180- and 200-grain 30 caliber match bullets in a full-metal jacket type with solid tip and hollow base. After experimenting with hollow point versions of the same weights/shapes and noting the dimensional and accuracy improvements, they changed to hollow point versions. The difference in drag between the solid and hollow point versions was insignificant.
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Old 11-02-2006, 08:17 AM
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Re: Ballistic Coefficient (BC)

This year I am going to shoot Barnes X Triple Shock bullets 168GR in my .300 win mag. I was on the Barnes site quit awhile back and noticed that the BC had changed, so I phoned and taked to a guy there at Barnes and he told me that Barnes changed the ogive of the bullet, which will make the bullet alot more accurate. I questioned him about the low BC and he told me the BC had nothing to do with accuracy. He said that with the change in the ogive of the bullet, the bullet will be more accurate, and that is what he said Barnes is striving for.
Both myself and my buddy are shooting 168GR Triple shocks out of our .300 win mags, and the accuracy is pretty good. I just can't wait to shoot something with my new gun and bullet combo.
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  #6  
Old 11-02-2006, 08:43 AM
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Re: Ballistic Coefficient (BC)

When testing a bullets' BC in flight, I would think that design of the tip and the base would be more critical than the edges of the bullet. When a bullet is fired the rifling will alter the edges.

When compairing a Ballistic Tip to a SMK, the Ballistic Tip "should" cut the air better than the hollow point. The boat tail design of each should reduce drag and turbulence behind the bullet. So in theory, the ballistic tip should be more accurate. But, we know this is not true.

Is it because of the interior construction of the bullet?

Rogue
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  #7  
Old 11-02-2006, 10:26 AM
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Re: Ballistic Coefficient (BC)

Dave tooley did some testing some time back putting amax tips in some other bullets. he figured out that the tips produced a more consistant BC bullet to bullet, with a slightly higher bc for the batch compared to un tipped bullets.

This is also where all our information about meplat trimming started.

Part of the reason most of the tipped bullets won't shoot with the SMK and J4 bullets, has to do with the simple fact that these bullets aren't assemled with the same care that companies like Berger, and Sierra place on their match bullets. Add to that that things like jacket quality are built into the bullet by the mfg of the jacket, and the bullet assembler has little controll over it.

As to how other materials affect BC... they do it through changing the SD. If you could take a bullet shape, and hold it as the constant, any materials lighter than lead will lower the SD, and things heavier than lead will raise it. Higher SD with the same form factor will raise BC, and lower SD with the same will lower it.

As far as where in the bullet the lead and other materials are located... there is some effect to BC, primarily as it effects the relationship of the bullets Center of Gravity, in relation to its Center of Pressure. Im gonna keep thsi really simple... CP behind the CG, and the bullet will be drag stabalized, and will fly point forward without being spin stabalized. CP in front of CG, and the bullet will try to fly backwards unless its spun, and if its not spun fast enough it will tumble because it can't damp all the torque that was imparted to it by the rifeling... so iw will never settle into stable base forward flight. Now then, our bullets have the CP in front of the CG, and this means that as they encounter a croswind, the bullet will be deflected, but that deflection will also effect how it is being gyroscopically stabilized, and it is the additional frontal area that is created as a result of the bullet torque relative to its flight path that decrease measured BC.
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