Cat Whisperer,
I noticed that this bullet is to be cast while I am a were that some people like to mix casting and swaging and that there is a purpose and place for casting alone, this is a swaging forum. But I will, never the lease, endeavor to answer your question.
With the un-pleasantries out of the way, your question recalls a flood of bullets shapes and designs. I will give you the information that you need but I will leave the decision of the design in your capable hands.
There are three basically different types of ogives out of four basically different nose shapes:
1. Tangential ogive is a constant curvature of a circle of radius S and is tangent to the shank.
2. Secant ogive is a tangential curvature that is offset of the ogive slightly to one side and brings it back, shortening the nose without changing the curve. This means the ogive curve joins the shank at some slight angle, so it is no longer tangent to the shank.
3. Elliptical ogive is an elongated circle that the angle is constantly changing, has two focus points, and is tangent to the shank. A circle is an Ellipse that the two focus points are at the same place in the ellipse.
4. Cone is a straight angle from the shank to the tip of the point.
And their variations like the truncated cone, the hollow point, the open tip, the Sears-Haack, the 3/4 power-law, the paraboloid, and others. All these nose shapes exhibit different drag coefficients as the velocity increases from subsonic to hypersonic. But there is one trend in common to all shapes and that is the longer the nose is the lower the drag coefficient will be. But like all things there is a limit and a trade off.
All ogival nose shapes are generated by circular arcs of radius R. Rt is the radius of a tangent ogive nose whose length is the same as the actual nose. This might sound a little confusing but please bear with me and hopefully it will make since. A tangent ogive will have a nose of Rt / R = 1, while a conical nose may be considered to have an ogive with an infinite radius of Rt / R = 0 and a secant ogive will have a radius that lies somewhere in between of 0 < Rt / R < 1. At high supersonic speeds; a secant ogive of Rt / R = 0.5 will generate the lowest drag coefficient, this is for any tangent of 1 there will correspondingly a secant that the curve lies at 0.5, half way between the tangent and a cone, while the tangent shows the highest drag. For the low supersonic speeds the same holds true for the curves that lies in the secant ogive but the difference is not as pronounced. In the subsonic speeds the tangent of Rt / R = 1 shows a lower drag coefficient than either the conical or the secant.
The tip of a bullet makes a difference on the drag coefficient as well.
1. The more that the tip is truncated the more the drag increases while the effect of small bluntness, meplat diameter less than .1 caliber, is insignificant.
2. The effect on drag of blunting the projectile nose by opening up the nose contour while maintaining the length at supersonic speeds, for small opening of the meplat gives lower drag than the sharp point nose. The melpat diameter that gives the lowest drag varies with the flight Mach number, the nose length, and the nose shape but in general a Mel pat diameter of 0.10 to 0.15 calibers is a very good choice over a wide range of Mach numbers. At supersonic speeds, an increased hemispherical tip, instead of a melpat gives a slight further reduction in drag.
The shank of any bullet should be at lease one caliber in length but that is normally easy to accomplish.
Now we come to the tail end of our discussion along with our bullet, the boattail. The difference between a flat base and the boattail is insignificant at velocities above the speed of sound and at relatively short distances, within 300 yards. But at longer distances and were the velocities will drop below the speed of sound for the majority of the range the boattail becomes a very significant way to reduce the drag coefficient. While the sporting industry has popularized the nine-degree angle boattail, research has shown that a seven-degree angle will give about a two percent lower total drag. But a boattail steeper that ten degree angle will destroy the boattail effectiveness as a drag-reducing device. While it is true the longer the boattail is the lower the drag coefficient will be it is also true that the flight dynamic instability will increase. For this reason I would suggest that one would keep the boattail length not to exceed one caliber.
As you can see there is no one answer to the question of which shape has the lowest drag like there is no one bullet construction is the best for all shooting situation. I do hope this helps.
Donna :wink:
