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Posted: 9/30/2004 3:07:03 PM EDT
In reloading tearms, the BC of a bullet?
Link Posted: 9/30/2004 3:08:21 PM EDT
Its a measure of aerodynamics. Someone will be along shortly who knows more. Its like the coefficient of drag is for a car.
Link Posted: 9/30/2004 3:09:21 PM EDT
Ballistic Coefficient (BC) is basically a measure of how streamlined a bullet is; that is, how well it cuts through the air. Mathematically, it is the ratio of a bullet's sectional density to its coefficient of form. Ballistic Coefficient is essentially a measure of air drag. The higher the number the less drag, and the more efficiently the bullet cuts through the air. So for purposes of flying through the air efficiently, the bigger the BC number the better.

BC is what determines trajectory and wind drift, other factors (velocity among them) being equal. BC changes with the shape of the bullet and the speed at which the bullet is traveling, while sectional density does not. Spitzer, which means pointed, is a more efficient shape than a round nose or a flat point. At the other end of the bullet, a boat tail (or tapered heel) reduces drag compared to a flat base. Both increase the BC of a bullet.

For example, a Hornady 100 grain round nose 6mm bullet has a BC of .216; a Hornady 100 grain spire point 6mm bullet has a BC of .357, and a Hornady 100 grain boat tail spire point 6mm bullet has a BC of .400. All three of these bullets have a sectional density (which is the ratio of a bullet's diameter to its weight) of .242, because they are all .243" in diameter and weigh 100 grains. But the more streamlined bullets have a higher ballistic coefficient. They are the ones to choose for long range shooting where a flatter trajectory is important.

To illustrate the practical difference between these three styles of bullets, let's use Hornady's trajectory figures for the 100 grain 6mm bullets above. Starting all three bullets at a muzzle velocity of 3100 fps from a scoped 6mm rifle zeroed at 300 yards, the trajectories are as follows.

.243" 100 grain Round Nose (BC .216): -1.5" @ muzzle, +4.8" @ 100 yards, +6" @ 200 yards, 0 at 300 yards, -15.9" @ 400 yards, -46" @ 500 yards.

.243" 100 grain Spire Point (BC .357): -1.5" @ muzzle, +3.8" @ 100 yards, +4.7" @ 200 yards, 0 @ 300 yards, -11.1" @ 400 yards, -30.5" @ 500 yards.

.243" 100 grain Spire Point BT (BC .400): -1.5" @ muzzle, +3.6" @ 100 yards, +4.4" @ 200 yards, 0 @ 300 yards, -10.4" @ 400 yards, -28.6" @ 500 yards.

There is a pretty big difference in trajectory between the round nose bullet and the two pointed bullets, making it obvious why it is folly to choose a round nose bullet for long range shooting with a high velocity rifle like a 6mm Remington or .243 Winchester. Also notice the big difference in BC between the round nose bullet (.216) and the spire point bullet (.357).

But there is less difference between the trajectory of the flat base spire point bullet and the boat tail spire point bullet. The boat tail helps, but not nearly as much as the point on the front of the bullet. The boat tail bullet had .3" inch less rise at 200 yards, and 1.9" less fall out at 500 yards. These differences are real, but unlikely to make or break a shot at a big game animal. This is shown by the smaller difference in BC between the two pointed bullets, .357 for the flat base and .400 for the boat tail.

To further assess the importance of a boat tail, note these pairs of Speer spitzer bullets of the same weight and caliber. In each pair, the first bullet has a flat base, and the second has a boat tail.

.243" (6mm) 100 grain, BC .351
.243" (6mm) 100 Grain BT, BC .430

.257" (.25) 100 grain, BC .369
.257" (.25) 100 grain BT, BC .393

.277" (.270) 130 grain, BC .408
.277" (.270) 130 grain BT, BC .449

.308" (.30) 165 grain, BC .433
.308" (.30) 165 grain BT, BC .477

A list of pointed (spitzer type) hunting bullets considered a good bet for long range shooting in their respective calibers, with their ballistic coefficients, follows. Boat tail bullets are designated "BT," all other bullets have flat bases.

All of the figures that follow are taken from the Speer Reloading Manual Number 13. The same weight bullets from other manufacturers will have different BC's, because they are slightly different shapes. But these Speer numbers are as typical as any, and being from the same source they are useful for purposes of comparison.

.224" (.22) 55 grain, BC .255
.243" (6mm) 90 grain, BC .385
.243" (6mm) 100 grain BT, BC .430
.257" (.25) 100 grain BT, BC .393
.257" (.25) 120 grain BT, BC .435
.264" (6.5mm) 120 grain, BC .433
.264" (6.5mm) 140 grain, BC .496
.277" (.270) 130 grain BT, BC .449
.284" (7mm) 145 grain, BC .457
.308" (.30) 150 grain BT, BC .423
.308" (.30) 165 grain BT, BC .477
.311" (.303) 150 grain, BC .411
.323" (8mm) 150 grain, BC .369
.338" (.338) 200 grain, BC .448
.375" (.375) 270 grain BT, BC .429

Note that with bullets of the same weight and style, such as the 100 grain .243" and .257" bullets or the 150 grain .311" and .323" bullets, the smaller diameter bullet always has the superior BC due to its better sectional density.

As examples of very streamlined bullets, note the BC of these Speer match type bullets. These are all pointed hollow point, boat tail bullets.

.224" (.22) 52 grain Match, BC .253
.284" (7mm) 145 grain Match, BC .465
.308 (.30) 168 grain Match, BC .480
.308 (.30) 190 grain Match, BC .540

Note that in the .308 pair, the heavier bullet (which has the greatest sectional density) has the better BC. The extremely poor SD of the .224" bullet lowers its BC, even though its shape is similar to the others.

Which explains why .22 bullets drop so much at long range and are so subject to wind drift, compared to larger caliber bullets with superior sectional densities (and hence, BC's). At a MV of 3100 fps a .224" Speer 52 grain BTHP Match bullet zeroed for 300 yards has a 500 yard drop of -43.9 inches, not much better than the 6mm round nose bullet in our trajectory examples near the beginning of this article. An interesting subject, this ballistic coefficient, and worth paying attention to when you select a bullet (or a caliber) for long range shooting.
Link Posted: 9/30/2004 3:14:58 PM EDT
Link Posted: 9/30/2004 3:16:08 PM EDT

What is Ballistic Coefficient and How is it Calculated?

Simply stated, the ballistic coefficient is a measure of how well a projectile behaves in air. The ballistic coefficient is an important and useful concept that relates the drag deceleration of a given projectile to the drag deceleration of a standard bullet. The concept of the standard bullet and related ballistic coefficients was a major step forward, because otherwise the drag characteristic of every type of bullet fired would have to be measured individually – an impossible undertaking.

Near the turn of the century, many tests were made by the Krupp Company of Germany to determine the retardation or drop characteristics of so-called standard bullets. Soon after the Krupp data were published a Russian army colonel named Mayevski constructed a mathematical model for the drag deceleration of a standard bullet. The standard bullet was one inch in diameter, weighed one pound and had an ogive head of 8 calibers radius. Colonel James M. Ingalls of the U.S. army later used Mayevski's mathematical model to compute his now famous ballistics table.

The standard Krupp bullet proved to be such a good model for use in calculating the ballistics of most bullets used in sporting firearms, that today most of the major bullet companies use the Ingalls or similar tables together with test firings of production bullets to compute ballistic coefficients for their bullets. These coefficients are published in most of the major reloading manuals. Most ammunition manufacturers do not publish the ballistic coefficients of their bullets, but instead include ballistic charts in their sales literature and catalogs that show drop and remaining down range velocity for bullets used in each cartridge that they manufacture.

The larger the ballistic coefficient, the more efficient the bullet’s performance in air. It can be described as the ratio it’s sectional density to its coefficient of form, where sectional density is the weight of the bullet divided by the square of its diameter. It can be written as:

(1) C = SD/i = w/id2

Where C = ballistic coefficient
SD = sectional density
i = form factor
w = weight of bullet, lbs.
d = diameter of the bullet, in.

Coefficient of form, or form factor, is a mathematical number that relates a bullet’s shape, smoothness and shape at the base. The form factor compares the shape of a bullet being tested to the shape of a standard bullet used in a particular ballistic table. The ballistic table referred to in this discussion will be Ingalls table unless other wise noted.
No method is known to calculate and describe the bullet’s shape in numbers suitable for development of a mathematical formula. However, a chart of bullet shapes was developed by Edgar Bugless and Wallace H. Coxe, Ballistic Engineers of the DuPont Co. and included in a series of ballistic charts called "Exterior Ballistic Charts" published by E.I. DuPont De Nemours & Co. In order to use the chart the user slides a bullet along the chart until the shape matches the ogive or head radius of the bullet. This value is transferred to an accompanying table to determine the form factor, based on muzzle velocity and point of the bullet (normal point, hollow point or flat nose configurations). A few example typical form factors are given below:

Blunt projectile, cylindrical 2.30
Blunt projectile, Taper sides 0.6 Cal. 1.10
Head radius of 8.0 Cal M.V. under 2000 f.p.s. 0.65
Balls with M.V. under 1000 f.p.s. 2.00

Many bullet manufacturers used the form factor method to estimate ballistic coefficients for their bullets until development of the modern chronograph and other sophisticated electronic measuring equipment. Today, most commercial bullet manufacturers determine ballistic coefficients by measuring muzzle velocity and time of flight over a known range or measuring the velocity at two points over a measured range. Because the test bullets often deviate from the standard projectile model, the ballistic coefficient changes slightly with velocity. Most bullet manufacturers list one ballistic coefficient for each bullet. One bullet manufacturer lists three or four ballistic coefficients for each of their bullets, depending on the velocity of the bullet downrange.

The ballistic coefficients published by the bullet manufacturers are reported at standard conditions of temperature, elevation and humidity. In order to calculate an accurate trajectory for your bullet the standard ballistic coefficient must be corrected for shooting conditions at your location. Several of the loading manuals show how this is done or you can use one of the ballistic software computer programs to do this for you.

The Load From A Disk ballistics software program, as described at this web site, will allow you to calculate the ballistic coefficient three different ways. These are:

1. Calculation of ballistic coefficient from trajectory.
2. Calculation of ballistic coefficient from velocity at two points.
3. Calculation of ballistic coefficient from shape as described above.

This is the only ballistics program that will allow you to calculate the ballistic coefficient by three different methods. The shape method even allows you to print out the bullet shape chart to determine the correct ogive for your bullet. This program also lets you correct the ballistic coefficient to site conditions.

The method of calculating the ballistic coefficient from velocity at two points was actually used to calculate the ballistic coefficients of 19 different cast bullets for the rifle and 32 different cast bullets for the handgun. The results of these tests were published in Rifle magazines #66 and #70. Sixteen brands of .22 long rifle ammo were also tested and their ballistic coefficients were published in the 36th Anniversary Edition of Gun Digest.

Watch our web site for the next topic of interest "Trajectory and how it is used." Until then, shoot safely and know where your bullets are going.

Link Posted: 9/30/2004 3:16:43 PM EDT
[Last Edit: 9/30/2004 3:22:21 PM EDT by Gunzilla]
ETA: I guess my description was a little too short to fit in this thread...
Link Posted: 9/30/2004 3:52:05 PM EDT
That sa Gooder info than I we're ta post.
Link Posted: 9/30/2004 3:56:07 PM EDT
Slikker than snot.....
Link Posted: 9/30/2004 4:24:23 PM EDT
I just type in jacket weight and type,core weight and type, ogive, tip diameter and caliber of course.It gives options on what it is shot through(atmosphere). Corbin software rocks. I haven't even "shot" the bullet yet. At that point the speed is up to me, annd tells trajectory, whether it is safe to fire overhead of people, how high it goes. I'm not going to go into some details. Everything from soft wood to du. While it can't be precise on some of my hybrids, it's worth every penny. It also can be used to look at finished bullets. Melt one down and a little measuring is all it takes.
Link Posted: 9/30/2004 7:13:32 PM EDT
BC is a measure of the bullets drag, or how quickly it will slow down after being fired. This means a bullet with a higher BC than another bullet (of same caliber and weight) retains more of its orignal speed when it strikes the target. The reason that this is important is that wind deflection is reduced by the shorter time of flight by the bullet with the higher BC.

That is all there is to understanding BC!
Link Posted: 9/30/2004 11:50:11 PM EDT
[Last Edit: 10/1/2004 12:32:33 AM EDT by CMB69]
It is a mathematical game. Here is the simplest, and maybe best, explanation I have heard.
Imagine a "theoretical", or can I say "virtual", projectile that does not slow down as it travels through the air. It is the "standard" and has a B C of 1.
A real bullet or other projectile (which does slow down) is fired and actual velocity is measured at various distances. The velocity of the real bullet is divided by the "standard", (initial velocity) the result will always be less than 1. The more "streamlined" the projectile, the more velocity it retains, the higher the number or B C.
It is just a way of comparing projectiles to determine which retains more velocity as it travels through the air.
Link Posted: 9/30/2004 11:53:05 PM EDT
Some bullets have a BC higher than 1, right? I remember hearing that the AMAX .25 had a 1.05 BC.
Link Posted: 10/1/2004 2:50:38 AM EDT
1 is perfection I believe. Lost River bullets have shown BC of .7-.8 but you'llpay for them.
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