I think I posted a link to this somewhere, but here's a good explantion everyone here should read concerning bullet design, what alters BC, why, and other considerations as to accuracy etc.
The following was copied and pasted here from Corbin's BR article.
Why use a secant ogive at all? The secant ogive lets us fool Mother Nature just a little bit. We can shorten the axial length (linear, down the middle of the bullet) of the nose, so the bullet still fits our gun and still requires normal spin rate, but join the long thin nose to the shank at a slight angle that doesn't generally create a shock wave. If we keep increasing the angle, soon we have a secondary shock wave generated at Mach I (speed of sound) bullet velocity and above, and the advantage of the lower drag co-efficient is lost. One shock wave is bad enough, and you can't design around that...no point designing in a second one to add more effective drag. The problem with shock waves is that they are related to a variable, the speed of sound. This is not a constant value, but varies with effective air density (which also varies with moisture and barometric pressure). So, if you push the secant ogive bullet design to the edge on a good day, it may work fine, but as soon as the weather changes, the bullet follows a different trajectory because it now generates that secondary shock wave that was absent before, and the BC drops as a result.
As you may know, if you have a vast number of bullets of all calibers and shapes and weights, but they all happen to have the same BC, then firing them all at the same initial velocity will result in them all following exactly the same trajectory. A .17 caliber and a .50 caliber will follow the same trajectory if they have the same BC. It's the meaning of BC, the relative rate of retardation compared to some standard bullet. But having delved into this aspect of bullet design, it's time for a reality check.
When the only consideration is accuracy, issues such as the BC are secondary. There is an unfortunate tendancy among shooters who do not have a physics or engineering background to equate the ballistic coefficient with the accuracy of a bullet. That is, a high BC is sometimes thought to be necessary for one hole groups.
Ballistic coefficient is primarily a comparison of the drag or rate of velocity loss of a given bullet with some other bullet that is selected as a standard or 1.0 on the graph. It has no meaning in absolute terms. To say that one bullet has a BC of .830 and another has a BC of .345 only has meaning if we are absolutely certain that both bullets were compared to the same standard. This is not always the case, since the number has become popularized beyond its strictly mathematical usefulness and is now used as a marketing tool.
Assuming that the standard bullet is specified, then a comparison of BC values for two bullets would have meaning. It indicates how closely the samples perform in overcoming air resistance (usually compared to a 1-inch boattailed spitzer artillery projectile). All other things being equal, a higher BC is desirable, but not at the expense of accuracy. The BC is determined by multiplying a coefficient called the "Ingall's number", which represents the inverse drag of the air upon the bullet, times the mass divided by the square of the diameter.
The BC goes up with an increase in weight, down with an increase in diameter, and up with a lowering of the air drag on the bullet. If a person were to strive for high BC, here are some of the ways to accomplish that:
1. Use a high density material, such as gold, tungsten, iridium, or osmium. High density puts more weight into a smaller package, so that the mass is increased for a given diameter without making the length excessive. This has the disadvantage of increasing bullet cost. Powdered metals can be compressed with normal swaging pressures, but have lower density than solid or sintered metal. Osmium, which vies with iridium as the heaviest stable metal, can produce deadly fumes under certain conditions. Gold and iridium are safe to use but rather costly. EZ-Flo Micro-Fine Tungsten powder is available from Corbin in 7,000 grain, 35,000 grain, and 70,000 grain flasks as well as by 50-kg pail, and is used by a number of custom bullet firms and government groups.
2. Make the bullet longer for a given diameter. This adds mass, but has the negative effect of requiring faster spin rate to keep the bullet pointed nose first. More spin amplifies any imbalances in the bullet radial parameters, such as slight differences in jacket wall thickness. Accuracy can become far worse from either high required spin rate, or from a bullet becoming too long for the barrel twist and thus unstable. High BC can in this case lead to less accuracy.
3. Reduce the included angle of the shock wave for bullets traveling above Mach I (the speed of sound) by making the ogive more sharply pointed, and reduce the turbulence at the bullet base by using a rebated boattail (which avoids generating a ball of gas through which the bullet must travel, like a conventional boattail). A longer nose requires a higher minimum weight, and a longer counter-balancing shank, so that as the ogive becomes more and more pointed, the bullet becomes longer and longer, and eventually the required spin rate to stabilize it becomes excessive. In a single shot bolt action rifle, the physical length of the bullet nose isn't a factor in functioning of the gun (assuming the chamber and leade were designed with the long bullet in mind). But spin rate puts a limit on practical ogive length.
Typically, benchrest bullets for under 200 yard shooting don't need to exaggerate the BC, and can gain more by using the lowest practical spin rate. Therefore, a 6-S ogive would be entirely practical. Many clients choose a 7-S custom ogive, or the 8-S ogive. A few choose the ULD (Ultra Low Drag, developed by Corbin for military and Treasury department sniper applications many years before the very similar VLD popular with civilians). But even though the ULD is our design, we feel strongly that it is not appropriate for medium and short range (100-300 yard) target shooting simply because it forces the shooter to use a longer bullet than necessary, which in turn requires a faster twist barrel, which in turn exaggerates any jacket wall eccentricity. So why do it? Who cares about the BC, if you are not shooting in a gale wind, at 100-300 yards? If you can read the mirage and the wind flags like a high power shooter, then you can certainly take advantage of the slower spin that stabilizes a normal weight 6-S bullet.
But the ULD design will help at 500-1000 yards (and of course, with 50 caliber benchrest at 1000-2000 yards, it will become a necessity as soon as enough other good shooters catch on). At some point, the bad effect of more spin balances the bad effect of wind drift on a lower BC bullet, and you choose the lesser of the two evils. This is no different from other bullet design fields, where you are always choosing between two contradictory values and trying to balance their bad effects in order to get the most use from their good effects.
Here's the link to the whole article. http://corbins.com/benchrst.htm
[ 09-05-2003: Message edited by: Brent ]