Comparing Advertised Ballistic Coeffi cients
With Independent Measurements Emily Bohnenkamp, Bradford Hackert, Maurice Motley, and Michael Courtney United States Air Force Academy
ABSTRACT This article addresses the question
of ballistic coeffi cient accuracy. Ballistic coeffi cients of bullets are important be- cause under or over estimates of ballis- tic coeffi cients can dramatically impact predictions of long range trajectory, wind drift, and impact energy. This project compares ballistic coeffi cients advertised by four well-known bullet companies (Hornady, Nosler, Sierra, and Barnes) with those measured by an independent source (Bryan Litz). G1 and G7 ballistic coeffi cients were determined using calculations at the JBM Ballistics web site. Many published ballistic coeffi cients are signifi cantly different from independent measure- ments, with Nosler’s advertised bal- listic coeffi cients showing the largest overestimates. INTRODUCTION
This article compares advertised
ballistic coeffi cients (BCs) of major bul- let companies (Hornady, Nosler, Sierra, and Barnes) with ballistic coeffi cients measured by an independent source. The ballistic coeffi cient is the ability of the bullet to overcome air resistance in flight. Ballistic coefficients relate the drag deceleration of a projectile to that of a standard bullet. Bullets with higher BCs move through air more ef- fi ciently. BC also is the ratio of sectional density of the bullet to its form factor, where sectional density is the weight of the bullet divided by the square of its diameter. Accurate determination of ballistic coeffi cient is important for predicting long range trajectory, wind drift, and retained energy. Earlier work has shown that manufacturer claims of ballistic coeffi cients are sometimes significantly exaggerated (Courtney and Courtney 2009). In addition to com- paring manufacturer claims of ballistic coeffi cients with those determined by Bryan Litz, a well-known match shooter and an expert in aerodynamics, G7 bal- listic coeffi cients for a wide variety of
bullets also are presented. Bryan Litz measured ballistic
coeffi cients using a chronograph and acoustic sensors over intervals between the rifl e and target. When the bullet was fi red, a chronograph measured the bullet’s initial velocity. Acoustic sensors measured the time of fl ight between intervals. The fi rst of four acoustic sen- sors was positioned at the chronograph, and each subsequent sensor was placed 200 yards farther downrange out to 600 yards total. As the bullet fl ew past each sensor, the supersonic “crack” of the bullet registered and was recorded to
a single audio fi le which is essentially a ‘time stamped’ trajectory for each shot (Litz 2009). Litz typically shot fi ve bullets per bullet type to determine a ballistic coeffi cient for each bullet. The physical difference between
the G1 and the G7 ballistic coeffi cients is that the standard projectile of the G1 has a short nose, fl at base, and bears more resemblance to an old unjacketed lead black powder cartridge rifl e bullet than to a modern long-range rifl e bullet (Litz 2009). The G7 standard projectile has a long boat-tail and its pointed nose ogive bears a much stronger resem-
Style Diameter Mass SD Barnes Litz (in)
TSXBT 0.308 TSXBT 0.308 TSXFB 0.257 TSXFB 0.284 TTSXBT 0.308 TTSXBT 0.338
Litz Overestimate
(gr) (lbs/in²) G1 BC G1 BC G7 BC (%) 168 0.253 0.404 0.400 0.2
1.00
180 0.271 0.453 0.458 0.229 -1.09 115 0.249 0.335 0.328 0.164 2.13 175 0.31
0.417 0.406 0.203 2.71 168 0.253 0.470 0.445 0.222 5.62
225 0.281 0.514 0.507 0.253 1.38 Table 1: Litz and Barnes BCs. The average Barnes overestimate is 1.96%.
Figure 1: G1 BCs from Barnes and Litz plotted vs. sectional density, along with best-fi t lines.
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