I was looking around the Border Barrels website tonight, where they have an excellent calculator for figuring out the optimum Twist for a known bullet....

http://www.border-barrels.com/barrel_twist.htm

It turns out that they also have a fabulous Bullet Designer as well.... You can input various dimensions, the material specific gravity (eg. it uses 11.4 for lead), and it will calculate the bullet weight, draw a diagram of the bullet shape, show a graph of the Drag Coefficient, and give you the Ballistics Coefficient at every 0.5 Mach, up to Mach 3.... If the bullet weight is known, you can use that as well.... It will even give you the optimum twist rate for the bullet at each Mach number....

http://www.border-barrels.com/drag.htm

Here is an example of the drag curve for a .22LR bullet.... The Coefficient of Drag is plotted against the Mach Number.... Notice what happens to the drag at about Mach 0.8 (900 fps).... Between there and about Mach 1.05 (1200 fps) the drag increases by about a factor of four....



The calculator also gives the Ballistics Coefficient, expressed as the G1 model.... For that bullet, it is 0.11 at Mach 0.5, 0.08 at Mach 1-2, and 0.07 at over Mach 2.... Changing the shape of the bullet significantly changes the shape of the drag curve.... Here is the graph for a 63 gr. bullet in .224 cal, with a .11" Meplat....



Note that the Drag Coefficient is roughly the same as the first example in the subsonic region (~0.23), but the inflection point where the drag increases is moved up to about Mach 0.9 (~1020 fps).... In addition, the supersonic drag is much less than for the round nosed .22LR bullet.... The G1 BCs for this bullet are 0.16 at Mach 0.5, 0.21 at Mach 1, 0.18 at Mach 1.5, and 0.17 from Mach 2 and up.... In other words, this shape is MUCH better at supersonic velocities, even though it has a flat nose....

You may wonder why the two bullets have such a large difference in BC when subsonic, even though the Drag Coefficient is similar at 0.23.... The answer lies in the Sectional Density.... They are the same diameter, but the 63 gr. bullet is 54% heavier, and has an SD of 0.179, compared to 0.118 for the 41 gr.... The drag is the same, but the heavier bullet has 54% more weight preventing it from slowing down.... and so the BC is also about the same amount higher....

Now for the interesting part.... I ran a whole bunch of cast bullets through this calculator, Round Nosed, Hollow Points, and those with varying sizes of Meplats, short dumpy bullets and long skinny ones.... The graphs showed the range of the Drag Coefficient at subsonic velocities was VERY small.... Most of the bullets by far had a subsonic Cd of ~0.23, and the range was between 0.22 and 0.25 for all the ones I looked at.... and they all showed minimum drag at Mach 0.5 or just below (~500 fps) In addition, all the bullets had a huge increase in drag at between Mach 0.8 and 0.9 (900-1020 fps).... Round Nosed bullets tended to get draggy at 900 fps, and the bigger the Meplat (in cailbers) the faster they could be driven before the big increase in drag started.... Short bullets (again, measured in calibers) tended to get draggy at 900 fps, and long ones more towards 1000.... which makes perfect sense as short bullets are lighter, and have a lower SD.... However, ALL the bullets showed a HUGE increase in drag at between 900-1000 fps.... one more reason to limit the velocity to that range....

If you are curious about how changing the shape of a bullet affects the drag, and want to get a good idea of the Ballistics Coefficient for a given design and weight.... this is an excellent website.... One more thing.... There is a button on the results page that says "Ballistics".... Click on that and you can put in the velocity and range and it will print out a drop and windage chart for you.... I don't think it gets any better....

Bob