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calculating bc

 
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  #1  
Old 12-29-2010, 07:24 AM
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calculating bc

i have looked all over and cant find it. is there a search somewhere on this website? maybe im just blind. anyway, how do you calculate the true ballistic coeficient of a given bullet. i know theres given data for many bullets but ive heard that the bc changes for different conditions, velocity and rifles. also if you trim the meplate on a bullet such as the sierra matchking, the bc will change. can someone tell me a way to find the true bc of a given bullet. do you messure velocity at different ranges, or drop from one range to another? please help me out here. thanks in advance.
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Old 12-29-2010, 07:31 AM
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Re: calculating bc

The formula for calculating the ballistic coefficient for bullets only is as follows:[1][2]
where:
  • BCBullets = ballistic coefficient
  • SD = sectional density, SD = mass of bullet in pounds or kilograms divided by its caliber squared in inches or meters; units are lb/in2 or kg/m2.
  • i = form factor, i = ; (CG ~ 0.5191)
  • CB = Drag coefficient of the bullet
  • CG = Drag coefficient of the G1 model bullet
  • M = Mass of object, lb or kg
  • d = diameter of the object, in or m
This BC formula gives the ratio of ballistic efficiency compared to the standard G1 model projectile. The standard projectile originates from the "C" standard reference projectile defined by the German steel, ammunition and armaments manufacturer Krupp in 1881.[3] The G1 model standard projectile has a BC of 1.[4] The French Gavre Commission decided to use this projectile as their first reference projectile, giving the G1 name.[5][6]
A bullet with a high BC will travel farther than one with a low BC since it will retain its velocity better as it flies downrange from the muzzle, will resist the wind better, and will “shoot flatter” (see external ballistics).[7]
When hunting with a rifle, a higher BC is desirable for several reasons. A higher BC results in a flatter trajectory which in turn reduces the effect of errors in estimating the distance to the target. This is particularly important when attempting a clean hit on the vitals of a game animal. If the target animal is closer than estimated, then the bullet will hit higher than expected. Conversely, if the animal is further than estimated the bullet will hit lower than expected. Such a difference in bullet drop can often make the difference between a clean kill and a wounded animal.
This difference in trajectories becomes more critical at longer ranges. For some cartridges, the difference in two bullet designs fired from the same rifle can result in a difference between the two of over 30 cm (1 foot) at 500 meters (550 yards). The difference in impact energy can also be great because kinetic energy depends on the square of the velocity. A bullet with a high BC arrives at the target faster and with more energy than one with a low BC.
Since the higher BC bullet gets to the target faster, it is also less affected by the crosswinds.

(http://en.wikipedia.org/wiki/Ballistic_coefficient)

I like math but I'd leave this to the pros - ballisticians. Personally, I'd go with the published BC (and use programs like what's on this site) and play around with the actual performance, drops, mV, and other factors you want to measure out of your tweaked loads at varying distances. I'm sure the pros will chime in soon.

Good luck!

Ed
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Last edited by FEENIX; 12-29-2010 at 07:41 AM.
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  #3  
Old 12-29-2010, 07:42 AM
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Re: calculating bc

I have not been to the site lately and am working from memory, but I recall that the JBM site has a section that allows you to input velocity at distances ( and possibly other variables) and back out a BC.

JeffVN
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Old 12-29-2010, 09:27 AM
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Re: calculating bc

FEENIX,

A slight history correction to the info you posted from Wikipedia; The Krupp (1881) and Gavre Commision firings (1873-1898) were the first "modern" attempts to determine the effects of atmospheric resistance on the new high velocity jacketed projectiles which were coming into being at that same time frame. Interesting period for ballistic research. Anyway, these and several other studies around the world were all using a roughly similar projectile for most of their firings, a blunt (2-caliber ogive) bullet approximately 3 calibers long, around one inch in diameter and weighing approximately one pound. They were generally refered to by the name of the commision doing the firings, i.e., Krupp, Gavre, or later the newer tables based on these earlier works, such as Russia's Mayevski, or our own Ingalls models. These newer works were also based around the same projectiles used in the earlier firings.

The problems associated with doing the workups on the entire range of different projectiles (including small arms, artillery and even bombs) wre greatly simplified witht he development of computers which could handle the tredious calculus problems involved, and that's when the work really took off. Our own Aberdeen Proving Ground developed a series of drag models for various shapes and properties during the late forties and fifties. Winchester's E.D. Lowrey published a compendium of these tables in 1965, which utilized the series of drag models we still use today; the G1, G2, G5, G6, G7, GL, etc.. The "G" used in the designation for this series was an homage to the work done by the Gavre Commision, but I'm not entirely sure who gave them the moniker. There's a good writeup of this in Hatcher's Notebook, by MG Julian Hatcher. Hatcher was the head of Army Ordnance for many years, and had something of a front row seat to many of these developments, making his perspectives particularly interesting. In that same book, he also has a complete set of Ingalls tables for calculating Ballistic Coefficients using the Ingalls' drag model. This table, incidentally, can be used with the later G1 data and BCs virtually interchangeably, with negligable errors in the final results.

Yeah, I know, I'm a nerd, but things like the comments in Wikipedia drive me nuts!
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  #5  
Old 12-29-2010, 09:55 AM
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Re: calculating bc

arrow, I don't believe there exists an 'easy way' to predict BC(no rules of thumb). Nor is there an easy way to measure it, or even express it!

The BC formula FEENIX brought in seems simple because it represents only a portion of work involved(the end calc). The "CB" in that formula(or CDtotal, or actual bullet total drag -at your velocities and conditions) does not just fall out of the sky into anyones hands...
Try a calc here for CD to get an idea-> JBM - Calculations

Here you'll find roadblocks like 'ogive radius' which is unknown and not easy to measure & determine with secant ogives. You'll find that meplat diameters matter pretty quick, and that they are difficult to measure with precision needed. Then there is more hidden work, as any BC figured has to be adjusted for air density and muzzle velocity..

And measuring BC is no simple task..
And correctly using/expressing the BC you hold is no simple task..
Yadda, yadda
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  #6  
Old 12-29-2010, 03:59 PM
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Posts: 101
Re: calculating bc

alright, so do you just start with the published data, typing that into the ballistic calculator and see how accurate it is? if i had a kestral 4500 and put those atmospheric conditions all into exbal and did a 100 yard zero then shot 300 and just plug in bc numbers until it matches. then try different distances? is this reliable?

if not, what im looking for are numbers people are plugging into exbal. i asume for the smk 175 .308 you would want g7 bc. is this correct? if so what numbers are you using? it will be coming out of a .308 winchester with a 20" barrel. the rifle is being built right now so i dont have velocitys yet. thanks
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  #7  
Old 12-29-2010, 05:31 PM
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Posts: 373
Re: calculating bc

Sometimes things just seem to get too complicated. Someone once said "keep it simple, stupid". I am not too big on math, but I think alot and am somewhat partial to John Porters way of calculating BC. For your initial setup use the data or BC supplied by the manufacturer. These are about all listed in the selection chart with most programs. Shoot your data with the inputs that you have, ie. altitude, temperature, aproxamate muzzle velocity. A chronograph is nice but not a necessity. Most reloading manuels will give something to start with. Now print a chart to use as a guide to figure your real BC and velocity. Set up dead on 0 at 100 or 200 yards. Shoot at 500, 700, and 1000 yards, using the printed chart as a guide. Your gun will probably shoot higher or lower, it doesn't matter , what is important is the data. If you are higher at 1000 than the chart, two things are possible, BC for your bullet is higher or your velocity is higher. Lower than the chart is just the opposite. Now that you know the number of clicks that it takes for your gun at 1000 and are dead on at 200 ,go back to the program and plug in your numbers, floating the velocity. This is very important, at this point we don't want to change the BC. Now print a new chart and it will be dead on at 200 and 1000.( you just put that info in to the chart) Now using your data from 500 and 700, compare them to your new chart. Hopefully they will be very close. If they are both high, your BC is slightly lower and can be tweeked down slightly till your data and that on the chart match. The opposite is done if those two middle numbers are lower than the chart. What we are trying to do is make a chart that matches your data exactly. I never float the BC but always float the velocity.I usually tweek the BC about .010 at a time. If the BC is 620 , I would go to 610 or up to 630. Once you find the matching data you are ready to print the final chart and get your turret printed. That is how I do my guns and scopes and it seems to work very well. The final velocity may vary from the chrono but if the data fits the chart, ballistics don't lie!!

Last edited by paphil; 12-29-2010 at 05:34 PM.
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