OK, so now we have a way to calculate the BC, if we know the Cd and the SD of a given projectile.... All we need to do is compare the Cd of our bullet with that of a standard bullet THAT HAS THE SAME SD, and at the same velocity.... They have to have the same SD or you would be comparing apples to oranges.... Remember;

BC = SD / FF .... so if you have an SD of 0.10, if the FF is the same as your standard projectile, the BC would be = 0.10 / 1.00 = 0.10.... Let's pick a bullet with an SD of 0.10.... that works out to 34 gr. in a .22 cal.... or it might be 44 gr. in .25 cal.... or 63 gr. in .30 cal.... or 86 gr. in .35 cal.... or 142 gr. in .45 cal.... It doesn't matter, as long as the SD is 0.10.... If the FF is the same as our standard projectile, then at a given velocity the Cd of our test bullet is the same as the Cd of the standard.... Effectively, this means that for any given velocity, the FF of our test bullet is Cd(test) / Cd(standard).... If our test bullet has less drag at that velocity, its FF will be less than one, so its BC will be greater than the standard bullet, providing they have the same SD and velocity.... If our test bullet has more drag than the standard, the FF will be greater than one, so the BC will be poorer than the standard bullet.... When FF = 1.000, then BC = SD, a very handy thing to remember....

We measured the velocity decay, and at an average velocity of 900 fps we determined that the Cd of our test bullet was 0.30.... I'll repost the chart here for convenience.... The black dot is our test bullet....

OK, so a quick glance at that graph shows that our test bullet has less drag than a cylinder or sphere (GC or GS).... but more drag than any of the other standard projectiles.... If they were all the same SD, then relative to each different standard, our test bullet would have a different FF.... and therefore a different BC.... depending on which standard we compared it to.... In fact that is exactly the case.... In the table below are the BCs of our test bullet, calculated for each of the different standards.... Remember, all these widely varied BCs are for the SAME bullet, just compared to a different standard.... In every case the SD = 0.10....

**Model BC**GC 0.310

GS 0.220

G1 0.085

GL 0.085

GA 0.080

RA4 0.078

G0.2 0.067

G7 0.041

Now you can see why knowing the BC means nothing if you don't know what standard was used to calculate it.... It gets even worse.... Let's say you are shooting a roundball, so you should be using the GS standard.... If you used a BC of 0.220, and the GS standard, in ChairGun, you would get an accurate prediction of the drag of your roundball at other velocities.... so you would know how it performed as it slows down going downrange.... However, let's say you used the G7 drag model, and a BC of 0.041.... Although the data would be correct at 900 fps, as the velocity decayed as your roundball went downrange, the calculations would be off, because the G7 profile says the Cd is constant below 900 fps, but in fact if you look at the GS profile you will see that the drag drops about 25% as the bullet slows down.... If you used a BC of 0.041, with the G7 profile, for your roundball, by the time it had slowed to 600 fps you would be using too low a BC, so the trajectory would actually be flatter than what you calculated....

The purpose of this thread was not to make you fearful of using BCs because they are useless, not at all.... It was to hopefully help you understand why it is important to choose a drag profile that closely matches your bullet, if you wish to get accurate data at widely differing velocities than where you measured the BC.... However, if you are interested in what is happening at a specific distance, and you measure your BC at that distance, you are good to go.... It is VERY important, however to know what Drag Model was used to calculate the BC.... If you calculate it using, say, the G1 profile, and then try and look at the trajectory using the GS profile, you will get very wrong answers.... The BC you calculate will vary widely depending on the Drag Model you pick.... In the above example, the BC is anywhere between 0.041 and 0.310.... but remember, that in this example, at 900 fps with SD = 0.10.... BC(GC) of 0.310 = BC(G7) of 0.041....

I hope I didn't confuse you totally....

.... I will now unlock this thread to answer any questions you have, but please stay on topic....

Bob