Sierra's Infinity Ballistics Program
Date: Jul 06 2010
There are three studies of ballistics: interior, exterior and terminal. The bullet has an influence on all three, but the easiest to understand, measure or categorize is external ballistics.
According to the NRA’s Firearms Source Book, interior ballistics is the branch of ballistics dealing with all aspects of the combustion phenomena occurring within the gun barrel, including pressure development and motion of the projectile along the bore of the firearm. The bullet’s bearing surface and the material the bullet is constructed from influence internal ballistics. Terminal ballistics deals with the effects of projectiles on the target such as penetration, deformation and tissue destruction. The way a bullet is made and the materials it is made from have a great deal to do with its terminal performance.
This same book defines exterior ballistics as the branch of ballistics dealing with the motion of a projectile from the muzzle to the target. External ballistics focuses on trajectory, wind drift and retained velocity.
Stated simply, external ballistics is the science of hitting the target. In order to hit where we aim, we must know how far the bullet will drop at the range the target is positioned, and we must know how much the wind will move the bullet before it gets there. Most ammunition manufacturers offer ballistic tables either online or in the form of a chart in a catalog. And, most handloading manuals provide some means for estimating the external ballistics for a particular bullet launched at a particular velocity.
The problem with ammunition manufacturers’ catalogs is they are based on a velocity that may or may not be relevant to that produced by your firearm. As for load manuals, the exterior ballistics data is based on some increment of velocity and wind speed. Both will get you close but neither is exact. You will also find that you may need several different manuals to get all the necessary data. And finally, if you have developed a wildcat cartridge, you might not find a velocity that matches the bullet you need data on.
Most of us own a computer, and if you beat around on the Internet long enough, you can probably find anything you are looking for, Time is the one commodity most of us have little of, and a source for external ballistics data in one easy-to access place will save time, allow us to expand our knowledge of exterior ballistics and make valid comparisons of our handloads.
Exterior ballistics is based on several things, including velocity, the ballistic coefficient of the bullet and the atmospheric conditions. Sierra has created a powerful computer program called Infinity V6 that has the ability to generate this ballistic information while taking all these factors into consideration. The program computes all essential elements of the trajectory, in one-yard (or meter) increments, of any bullet that has a ballistic coefficient referenced to the “G1” drag function, and for any set of firing conditions that include muzzle velocity, elevation, altitude, wind and weather conditions.
The trajectory parameters calculated by the program include velocity, energy, momentum, bullet drop, bullet path relative to line of sight and time of flight. All are computed from the muzzle as the bullet moves downrange. The program outputs a list of all these parameters and provides graphs of velocity, energy, drop, bullet path and crosswind drift. Results are exportable and printable.