What is A Ballistic Coefficient?

A bullet that leaves the muzzle of your rifle must immediately do battle with natural forces.  First, it is acted on by the force of gravity, which accelerates its fall toward the earth’s surface.  Second, it must do battle with the atmosphere whose molecular nature creates a drag that impedes the bullet’s forward progress.  The bullet will lose these battles, the battle with gravity first if it does not encounter an obstacle in its path.  The drag of the air will slow the bullet and allow gravity to bring it to ground much closer to the muzzle than if the drag did not exist.

There is nothing to be done about gravity, as long as our shooting range is on earth.  Galileo showed that all objects, regardless of weight, will be drawn down to ground at the same rate, and Sir Isaac Newton presented the mathematics so that we can calculate forces and accelerations based on masses and the distance between them.  No matter what the weight or the velocity, bullets fired parallel to the earth’s surface are going down at the same rate.

The drag of the atmosphere is different.  Unlike gravitational force, the drag force depends upon the weight and shape of the bullet.  This means that we can have some effect on bullet performance by controlling its weight and shape.  Take a look at the three thirty-caliber bullets in the picture.  The two round-nosed (RN) bullets are of similar shape but the first weighs 180 grains, the second, 220 grains.  The third has a spire point and a boat tail (SPBT) but weighs only 165 grains. 

Left, 180-grain Round Nose; Center, 220-gr Round Nose; Right, 165-gr Soft Point Boat Tail

Suppose these three bullets are launched at the same velocity, 2500 feet-per-second.  They will lose velocity due to atmospheric drag as they travel downrange, and at 300 yards from the muzzle ballistics tables tell me that their velocities will be 1654 fps, 1789 fps, and 1886 fps, respectively.  The heavier RN holds its velocity better than the lighter RN, but the SPBT is the winner, in spite of being the lightest of the three.  We could say that it is the most efficient bullet of the three in retaining 232 fps more velocity than the 180-gr RN.  This leads to a flatter trajectory and less drift in a crosswind.

Comparing only the round-nose bullets, it seems to make sense that the heavier bullet would hold its velocity better and this, in fact, is always found with bullets of similar design.  The performance of the lighter SPBT, however, shows that something other than weight is involved and that something is bullet shape.  It is obvious that the SPBT bullet is more streamlined and that invites us to suppose that it would have less drag and hold velocity better.  A more precise analysis results from consideration of the bullet’s “Ballistic Coefficient,” a descriptive number that takes a bullet’s weight and shape into account and expresses the bullet’s ability to resist atmospheric drag and thereby retain velocity.  Most all loading manuals have a discussion of ballistic coefficients and all bullet makers publish the values of the coefficients for the bullets in their line.  Velocity loss downrange can then be determined for any commercially available bullet.  Loading manuals may thus include tables showing downrange velocity and trajectory data for bullets launched at different initial velocities.

The Ballistic Coefficient is actually a ratio which compares the velocity performance of a given bullet with that of a “standard” bullet.  The velocity performance of the standard bullet can be calculated.  The standard bullet is very good at air bucking, so BC values for available bullets are mostly fractions.  They vary from about .10 to .60, and the higher the number, the better the velocity retention.  The ratio nature of the BC definition simplifies its determination for a new bullet.  Velocities down range for a bullet of known initial velocity can be measured to provide the needed data.  If all bullet makers use the same data for the “standard” bullet, and I think they do, then BC values will be comparable across the board.  It is tempting to think of the “standard bullet” as a “perfect bullet” and then the BC value would be a measure of performance relative to perfection.  This is not the case as the standard bullet is not quite perfect.  Some long, heavy, highly streamlined bullets have better than standard performance and therefore have BC values above 1.00, but this is of no importance to hunters or target shooters.

Getting back to the three bullets in question (described above), I could have predicted their relative velocity retention from their BC values:  180-gr RN, BC = .28;  220-gr RN, BC = .34; 165-gr SPBT, BC = .40.  Note that I mean I could have predicted the rank of their downrange velocities, not the actual values.  For actual values I would need the ballistic tables that bullet makers provide in their loading manuals.

So, in the end, how much attention do I pay to the Ballistic Coefficient in choosing bullets for various purposes?  Let us point out, first of all, that a higher BC does not mean that a bullet will be a better performer in terms of accuracy or performance on game, because that is not what the BC measures.  There are lots of bullets of relatively low BC that are superbly accurate and give fine performance in killing game.  For long-range shooting, however, you want the velocity retention that a high value of BC will give.  Not mentioned to this point is that the higher BC will also minimize the bullet’s susceptibility to wind deflection.  Higher velocity, flatter trajectory, and less wind deflection all recommend the high-BC bullet for long range shooting.

For a simple example, suppose I want to choose a good bullet to try in my .22 Hornet at the normal Hornet velocity of 2600 fps.  Consulting various loading manuals, I find that my desired velocity is easily attained but that available Hornet bullets have BC values in the range .11 to .24.  This difference could lead to a velocity difference at 200 yards of more than 500 feet per second!  If I want my Hornet to have enough smack to take ground hogs at range, then I am going to have to take bullet weight and ballistic coefficient into account when making a choice.

Most of my own projects involve shooting groups at 100 yards or less and so the Ballistic Coefficient is not a large factor in my choice of bullets.  I have found many round-nose and flat-nose (in the case of .30-30) bullets that are superbly accurate at these and even longer ranges.  Hunters need not worry about BC out to 200 yards, but for 300 and longer, other things being equal, a bullet of higher BC should be chosen.

Can you be a fine target or game shot without knowing anything about Ballistic Coefficients?  Absolutely!  Choose ammo known to have good bullets for your application, sight in, and blaze away!

 

 

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One Response to What is A Ballistic Coefficient?

  1. Roger Roger says:

    Excellent article. Thanks for the great explanation. This is a great site with interesting articles.