Stability
Since the stability of a bullet is so strongly dependant on atmospheric conditions, SG values are tabulated for 3 different conditions. The Best, Nominal, and Worst case scenarios referred to in the stability table correspond to the following set of conditions:
- Best case: 1000 feet altitude, 100% humidity, 100 degrees F, and 3000 fps muzzle velocity.
- Nominal case: sea level, 50% humidity, 59 degrees F, and 2800 fps muzzle velocity.
- Worse case: sea level, 0% humidity, 0 degrees F, and 2600 fps muzzle velocity.
The description of Best case is arguable since the higher you go in altitude, the more stability will improve. Just note that if you're at a higher elevation than 1000 feet, that the gyroscopic stability factor (SG) of your bullet will be even higher than that indicated by the Best Case in the bullet data. This Best Case is really just to show how much stability is improved at just 1000 feet altitude.
Ogive metrics, and Rt/R
The ogive of a bullet is usually characterized by the length of its radius. This radius is often given in calibers instead of inches. For example, an 8 ogive 6mm bullet has an ogive that is a segment of a circular arc with a radius of 8*.243 = 1.952”. A .30 caliber bullet with an 8 ogive will be proportionally the same as the 8 ogive 6mm bullet, but the actual radius will be 2.464” for the .30 caliber bullet.
For a given nose length, if an ogive is perfectly tangent, it will have a very specific radius. Any radius longer than that will cause the ogive to be secant. Secant ogives can range from very mild (short radius) to very aggressive (long radius). The drag of a secant ogive is minimized when its radius is twice as long as a tangent ogive radius. In other words, if a tangent ogive has an 8 caliber radius, then the longest practical secant ogive radius is 16 calibers long for a given nose length.
Tangent ogive Secant ogive
There is a number that’s used to quantify how secant an ogive is. The metric is known as the Rt/R ratio and it’s the ratio of the tangent ogive radius to the actual ogive radius for a given bullet. In the above example, the 16 caliber ogive would have an Rt/R ratio of 0.5. 0.5 is therefore the lowest practical value for the Rt/R ratio, and represents the minimum drag ogive for a given length. An ogive that’s perfectly tangent will have an Rt/R ratio of 1.0. Most ogives are in between an Rt/R of 1.0 and 0.5. The dimensioned drawings at the end of this book provide the bullets ogive radius in calibers, as well as the Rt/R ratio. In short, the Rt/R ratio is simply a measure of how secant an ogive is. 1.0 is not secant at all, 0.5 is as secant as it gets.
