What is BC?
The Ballistic Coefficient or BC is a term from the Siacci Method of calculating trajectories. Developed by Colonel Francesco Siacci of Italy in 1880, this method has been used for over a hundred years as the primary method of calculating trajectories before the development of ballistic computers. It is still the taught in various sniper schools as a backup method in the event of ballistic computer failure.
The Siacci Method separates the relatively complex math of integrating the drag curve (a calculus term) from the relatively simple math of bullet drop, time of flight, and impact velocity. This allows the shooter to calculate bullet trajectories with a simple notebook by hand as long as the shooter has a Siacci Table that is based upon the bullet drag curve. The Siacci Method has a term that shows up repeatedly, the bullet mass divided by the diameter squared, that was named the ballistic coefficient.
Calculating the Siacci Tables is not a simple task and it would be nice to use the same table with multiple bullets. To expand the Siacci Method to bullets other than the one that was originally calculated, a method called shape factor was developed. If you multiply the ballistic coefficient by the ratio of the drag curve of the original bullet by the drag curve of the new bullet you are able to use the Siacci Tables from the old bullet with the new bullet pretty accurately.
Finally, to simplify the method over a larger range of bullets and guns, The US Army Ballistic Research Lab created a set of standard projectiles (also called the general projectiles). Some were more pointy and sleek while some were more blunt and rounded. The shooter can now calculate the BC of a new bullet from a few test firing and use the Siacci Table for the closes matching standard bullet.
How to use the BC
The G1 Standard Bullet
The G7 Standard Bullet
The two most commonly used standard projectiles are the G1 and G7 bullet designs. If you’re are shooting a bullet that looks similar to the G7 bullet, you can use the BC compared to the standard projectile to enable you to use the G7 Siacci Tables.
The Sierra 168 grain BTHP
The Sierra 168 grain BTHP bullet is similar in shape to the G7 projectile and the drag curve for the Sierra bullet is similar in shape to the G7 drag curve. We can get a good estimate of the trajectory of a Sierra 168 grain BTHP by using the G7 BC and the G7 Siacci Tables. The Sierra 168 BTHP looks nothing like the G1 projectile and you would not use the G1 BC for this bullet.
Aerospike BC
Aerospike Bullets are based upon the most advanced aerodynamic engineering theory. The ogive (or nose) of an Aerospike Bullet is based upon a solution of supersonic potential flow theory for minimum drag. The Aerospike base is based upon an inverted rocket nozzle design that is designed using a method called the supersonic Method of Characteristics. These advances were developed well after the work of the US Army BRL in the 1940’s.
The Aerospike Bullet drag curve does not match any standard projectiles.
The Aerospike Bullet drag curve is too different from standard projectiles for the use of the BC method. It is still possible to calculate the BC of Aerospike Bullets but the numbers are not accurate. Any ballistic calculator that is based upon the Siacci Method using standard projectiles as a reference would do a poor job of calculating an Aerospike Bullet trajectory.