B.C. or not B.C.; that is the ?

[Bigjohn]

In the sport/hobby that we take part in B.C. (Ballistic Coefficient) is an important number to have.

So, here comes a question I have and so would many other shooters about what to do if that number is not available. How do we calculate what the Ballistic Coefficient of a boolit we have just cast is, if the mould maker has not provided it?

Also, how much would the change of metal mix/weight affect this number?

I have seen articles on various methods; one involving the use of two chronographs; one for the muzzle vel. and one to collect the velocity remaining at approximately 100yds and these figures are used to calculate the B.C..

As I understand it, this number is what we use to compare our boolits performance (trajectory, wind drift, fpe etc) at "whatever" velocity we are able to launch them from the firearm against a known graph.

There is still the old tried and true method of shooting the firearm and adjusting the sights accordingly but that all changes if you alter one or more of the perfromance parameters.

Please note; I'm terrible a maths, which is way I'm possibly asking this question. I just hope to learn more.

John

[rhead]

http://www.handloads.com/calc/index.html
In the upper left portion of the screen there is a box containing a B C calculator. You can use it and the ballistic calculator to see what a change in bullet shape length or diamaiter would do to the B C , of what a change in B C would do to the flight path.

[Larry Gibson]

I've found the BC calculator at the Handloads site to give only a very rough idea of the actual BC at best, actually not much better than guessing based on a similar bullet in a manual with listed BCs. I measure BC via an Oehler M43 which records muzzle velocity and down range velocity (another set of screens is in front of the 100 yard target) and have compared numerous actual BCs of various calibers, jacketed and cast bullets. Only in a couple instances out of numerous comparisons has the Handload calculator BC been close enough to what I consider a useable BC.

If you can not actually measure the BC then I've found the best thing is to find a similar designed cast bullet (caliber doesn't matter) in the Lyman manual and use that BC. The windage and elevation data is also quite close using that method.

Remember that we se BC carried to the third decimal point but in reality that third decimal point number is like knowing the deer was at 174 yards instead of the 173 yards we thought when we shot it. In other words the third decimal point number is not relevant. Being within 1-2 numbers of the second decimal point is very close considering the practical shooting ranges of most cast bullet loads. Other variables such as temperature, humidity, wind direction, velocity variations, etc. have more effect on the trajectory at longer range than does a 1-2 variation of the second decimal number.

Larry Gibson

[Cap'n Morgan]

In case you don't have two chronographs, a single chronograph will do fine. Take readings from four or five shots at both distances and calculate the BC from the average velocity.

[HORNET]

Also remember that the B.C. is NOT a constant. One of the factors involved in the B.C. is the Drag Coefficient which varies with velocity. IIRC, one of the Lyman manuals (Cast Manual #3?) had the B.C.' s listed for some of their designs with useable approximations given for 3 different velocity ranges. This also screws people up in the real Greenhill Formula for rifling twist.