This is part of an article by Corbin:
When a flat base bullet flys through the air at any velocity, it displaces an equal volume of air which then rushes in behind the passing bullet to fill the vaccuum. This happens at any velocity. But at speeds below Mach I (speed of sound), the drag caused by this turbulence is greater than most of the other forces slowing down the bullet. When the sound barrier is crossed, the air is compressed faster than it can move out of the way, and it compacts into a dense wave that is dragged along by the nose of the bullet. Moving this compressed shock wave adsorbs far more energy than the turbulent base drag. Thus, at super-sonic speeds, the nose shape has a greater effect than the base shape on the total retardation of the bullet (as a percentage of total drag).
If the base of the bullet were made more streamlined, then the air would be put back together more smoothly, with less turbulence, and would fill the space left by the passing bullet more quickly. This would eliminate much of the base drag. Putting a point on both ends of the bullet accomplishes this, but it generally makes the bullet too long, so that it takes up too much powder space or causes other physical problems in a practical size cartridge case or gun. There is no technical reason why a double-ended bullet should not be used to overcome drag, except for these physical considerations.
The next best idea is to use a truncated conical shape on the base, so that the bullet has some degree of streamlining to help reduce base turbulence. The conventional boattail does this reasonably well. By having a base that is made of an angled portion of the shank, the size of the flat base is reduced so that the turbulence works on a smaller area. A typical, practical size of boattail has from nine to fifteen degrees (measured from the center-line of the bullet) and is about a caliber long. There is no great difference in the performance of any specific angle or length within this general range.
The boattail reduces base drag at all velocities, but has the greatest percentage of effect when the bullet is flying at sub-sonic velocity. Thus, it is more important for slow bullets, or bullets fired at long ranges, than for bullets which will be moving at super-sonic speed over their entire path to the target. Ideally, pistol bullets should be boattails. However, due to the length and weight limitations imposed on most pistol bullets, this superior design is rarely used for handguns. It is often applied to rifle designs where it will have relatively little effect, but is primarily effective for promotional purposes.
Full article here:
http://www.wildcatbullets.homestead.com/BulletTheory.html