Whilst I understand that all projectiles are given a ballistic co-efficient based on their shape,weight,diameter etcetera.

How is this ballistic co-efficient figure arrived at ?

Is there a formula I can use to work this out myself ?

Does ballistic co-efficient really have that much effect on velocity & therefore trajectory ?

It is a number, assigned to a hypothetical perfect bullet, under ideal and constant atmospheric conditions, at a constant velocity. There are three major considerations that we MUST look at here. "Perfect", "Ideal", "Constant". None of these apply to any man or environment know to man, or at least inhabited by man.

First, what is the "perfect" bullet? It is a bullet which is three-calibers long, and ogival head of two-calibers radius, and of homogenous construction with equal and concentricity of the mass around the center from tip to butt.

This bullet must be fired from a source that will establish and guarantee that a constant velocity of that bullet will remain from the moment of launch until the moment of impact.

In addition, all this MUST take place at exactly sea level, at a temperature or 59-degrees F., 29.58-inches of mercury barometric pressure and 78% humidity. Oh, and absolutely no movement of the air.

Now if we could find that "perfect" bullet, and launch it and maintain it at the "constant" velocity, under the "ideal" conditions, we would be able to assign a ballistic coefficient of 1.000 to that bullet.

All bullet manufacturers (other than Sierra) assign a B.C. rating to each and every one of their bullets under the "ideal" or "standard" conditions I have listed above. How do they derive their numbers? No, they don't travel to the moon, they do it on computers. Someone, somewhere computed how long it would take the bullet with the B.C. of "1.000" to travel a specified distance. For simplicity's sake, let's say it took one second. Then, they fired one of their less than ideal bullets the same distance and measured the amount of time it took that bullet to travel that distance. Let's say it took 1.3 seconds. That bullet would then rate a B.C. of "0.768".

morton3,
BC's are best derived through experimental means...meaning you should fire the bullet while measuring muzzle velocity and velocities at different yardages simultaneously.

There are fomula's you can use, but it's faster, easier and overall better to just use software such as Ristow's.

BC's can really have an effect on velocity and trajectory and the effect becomes more pronounced as the distance increases. Also remember that BC also effects wind drift.

In practical terms, Laser Rangfinders are cheap and quick. Ballistic program derived drop tables can put you on target once the range is known. But you still have to account for wind and there's no technology, but a better BC that can help you there.

ricciardelli,
You are mistaken. Your so-called perfect bullet is not a mythical "perfect bullet". They are "Standard Projectiles" of specific dimension and properties that were tested by actual firing and measuring performance. For instance the projectile dimensions you cite as the "Perfect Projectile" is known as the G1 Standard Projectile and is a modification of one of the projectiles used by Krupps, the large German ordnance factory in the 1880's. They collected data by actually firing your so-called "perfect" bullet among others at the Meppen Proving Grounds.

After analyzing the Krupps data, Mayevski developed a mathematical model for the ballistic performance for one of the Krupp projectiles. This projectile was the one with a 3.3 caliber length, 2 caliber ogival radius etc. This became known as the Mayevski Projectile. Mayevski produced a series of algoritms expressing retardation as a function of velocity, with each algorithm being valid for a small range of velocity.

American Colonel James Ingalls boiled down Mayevski's Algorithm into an easier to use table form.

BRL (Ballistic Research Laboratories) conducted research on a modified Mayevski Standard Projectile (along with 7 others) and came up with drag data for each. The modified Mayevski drag curve was designated G1 ("G" as an acknowlegement to the French Gavre Commision who had also done early work on Projectile Drag functions). There never was a stipulation of maintaining a constant velocity (as I'm guessing you're theorizing would be done in a wind tunnel). Actual flight data was collected at Aberdeen Proving Grounds.

Ballistic Coefficient is a comparison of how your bullet performs versus the G1 Standard Projectile. The G1 Projectile's BC is by definition 1.0 So a bullet with a BC of .500 performs roughly "half" as well as the G1 Standard Projectile.

If you're thinking "Hell, my 30 cal, 180gr Boat Tail Spitzer looks waaaay more aerodynamic than what the G1 Projectile sounds like", you need to realize that all the Standard Projectiles (G1 through G8) are all 1 pound in weight, 1 inch in diameter and solid lead (remember the original purpose was artillery not small arms)....and size (more correctly mass) does matter when you're talking BC.

Some of the other BRL Standard Projectiles such as the G7 and G8 more closely resemble the Spitzer BoatTailed bullets in common use today and their drag curves would be more appropriate to use for our purposes, but Bullet Manufacturers continue to use the G1 BC's (probably because the G7 or G8 BC's wouldn't sound nearly as impressive).

Here's a site that explains thing in a little more detail;
RSI on External Ballistics

Hey Morton3, Looks like Chris has added enough confusion to make the entire issue not worth bothering with. And he has tossed in a "misleading" statement for good measure.

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morton3,
BC's are best derived through experimental means...meaning you should fire the bullet while measuring muzzle velocity and velocities at different yardages simultaneously.

There are fomula's you can use, but it's faster, easier and overall better to just use software such as Ristow's.

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Absolutely no need at all to go through that fiasco.

The Ballistic Coefficient is simply a measurement of how easily a bullet's specific shape is able to fly through the air, it's ability to overcome air resistance. A higher BC means it gets to what ever you are aiming at "slightly" quicker and therefore with a bit more energy.

No need to calculate it since the BCs are listed in the Manufacturers Manuals. And those numbers are never exactly correct for real world use. Just round them up to the largest 0.X value and group them in your mind accordingly.

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... Ballistic program derived drop tables can put you on target once the range is known. ...

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I consider this a bit misleading for a person asking what BC actually is. But the software Chris mentioned as well as many others or Ballistic Tables in the back of your Bullet Manuals can "assist" in getting your initial "Paper Groups" on Target.

From there you need to develop your own Drop Chart by actually shooting your specific Load in your specific rifle, determine what the Point-of-Impact Drop is, and record those values for your Drop Chart.

The misleading part of Chris post, even though he didn't say it, is some people will get the totally wrong idea that it is OK to just sight in at 100yds, calculate the Drop on the computer, grab a lazer range finder and go hunting. That is not what you want to do at all.

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If you develop your own Drop Charts and use an adequate Cartridge, BC is of little value inside 400yds.

Well, let's try and not throw the sand out of the sandbox, shall we?

To answer the questions:

BC of a bullet is measured by measuring the velocity of a bullet at two different points (same shot). You can attempt to do it by measuring the drop and the velocity, but the accuracy of that method is rather poor.

You CAN calculate an estimation of BC yourself, but it is hard work for little reward. You can also use a site such as JBM's, that will do the calculating for you.

As far as "does it matter", the answer is yes, and no. To the 1,000 yard match shooter, BC is everything. To the 100 yard meat shooter, BC is irrelevant.

To see the effect of BC, I would suggest you go to www.huntingnut.com and download the free exterior ballistics program "Pointblank". It is simple to use, and will answer most of your questions by playing with different bullets. HTH, Dutch.

Really, truly, honestly, BC is one of those ideas a gun writer made up to meet a deadline. The idea has hung around long enough to cause arguments like this.

Don�t bother flaming me; I�m heading out to Colorado to shoot some BC incorrect bullets at a cow elk and a Mule deer buck. If I get shots, I wonder if either the deer or elk will feel the difference in BC of my bullets?

I use the BC data unless testing proves it off in some amount. The pessimism shown by Steve R. and what Hot Core wrote is not without reason. Some testing has shown that downrange velocities are not the same as what they calculate to. This throws data off of course.

To date I have found that the drop in the 350 yd range is really on using the generated data when a fast moving sleek bullet's 200 yd zero is known. This is useful for varmint shooting.

So to comedown on what side? I say to use the data and to verify it as soon as you can. What else to do?

Hi
Interesting thread..
I think we can all agree that what is on paper is on paper and what is reality is reality. What I mean is, if you're gonna be shootin at things at say 400 yards, and the ballistic table says you're gonna have a certain drop at 400 with a 200 yard zero with the bullet that you are using, then ya better start shooting groups out there at 400 yards so you can see that most of the time the center of these 400 yard groups are not exactly where they are supposed to be according to the ballistic table. And, you gonna find out that the center of the groups are gonna vary depending on weather conditions because your muzzle velocity varies with temperature. Field trails are the only way to be sure of this.
The number given to a particular bullet called Ballistic Coefficient is in practical terms a relative number to compare against another bullet. If you shoot two bullets at a 400 yard target, given the same rifle and load (the same muzzle velocity and both sighted in at 200 yards), one having a B.C. of .300 and the other having a B.C. of .500, the one having a B.C. of .500 will drop a little less than the one having a B.C. of .300. The larger the decimal number given to the bullet, or the closer the number is to the whole number '1', the less resistance the bullet will have against the air as it travels downrange.
The previous responses explain Ballistic Coefficient in scientific terms very well. I hope this explains it in practical terms.

Well, you have some explainations of BC is and where it came from. But it is over my head and doesn't serve much use to me.

I just deal with the observation that at the velocities of high power rifles, the standard BC (of 1.000) will get you 1/2 the starting energy at 1,000 yards.

This means that any given BC gives you the distance that you bullet will reach 1/2 its starting energy (that is when multiplying by the 1,000 yards) (or .71 of the starting velocity). This means that a 30-06 with 3,000 ft-lbs of energy at the muzzle will give 1,500 ft-lbs at 250 yards with a BC of .250. This will goto 450 yards with a BC of .450 with the same retained energy. etc.

It also can be done in reverse. Just look at any ammo chart from winchester, remington, federal (and others). Look across the energy charts to where the remaining energy equals 1/2 the muzzle energy and the yards (divided by 1,000) equal the BC for this round.


But do be aware that this energy loss is valid only for velocities above the speed of sound. Even there it will vary with its speed. But, below the speed of sound I have some listings that suggest the energy loss is only 1/3 of when its speed is above the speed of sound.


This doesn't account for the different manufactures comeing up with very optumistic numbers. As I remember, speer BC's looked to be higher than hornadys. Also the height (air density) and temperature make significant differences in the effective BC.

For me, all this suggests the BC (times 1,000) gives a first guess at the range which is appropriate for the bullet. This is a first guess as I know that black powder shooters extend far beyond this. Many varmint shooters also go well beyond this first guess.

In the end, shooting the ammo is the only way to be sure how good it is (I think that was said above, several times).


JerryO