Calculating Ballistics Coefficient

[North_of_60]

Is there a way of computing, or predicting an estimated, ballistic coefficient of a CB without actually firing it?

If not I have thought of setting my chrony at 10' from the muzzle and taking an average of 3 to five shots and then moving it out to 20' and taking another average then computing the difference. Does anyone know of a formula to do that?

Al

[Tom Myers]

Is there a way of computing, or predicting an estimated, ballistic coefficient of a CB without actually firing it?

If not I have thought of setting my chrony at 10' from the muzzle and taking an average of 3 to five shots and then moving it out to 20' and taking another average then computing the difference. Does anyone know of a formula to do that?

Al

Al,

You can estimate a reasonably accurate Ballistic Coefficient by entering the physical measurements of your bullet in this online BC calculator.

TMT Enterprises' Precision Ballistic Coefficient Calculator (http://www.tmtpages.com/calcbc/calcbc.htm)

Or you can calculate the coefficient with more precision using this software package.

G1 Ballistic Coefficient Estimator Version 4.1.0 (http://www.tmtpages.com/cast_bc.htm)

A precisely accurate Ballistic Coefficient for a particular bullet fired from a certain firearm needs either velocity values at different ranges or drop and sight setting values at different ranges.

The ranges do need to be farther apart than 20 feet though. more like 100 yards or even more as the error factor at short ranges make the values inaccurate.

A Ballistic software package is required to calculate a ballistic coefficient from either velocity or trajectory drop values as there is not a simple formula for that purpose.

A software package such as the P r e c i s i o n B a l l i s t i c s ~ V e r s i o n 4.1.0 (http://www.tmtpages.com/basbal/bal.htm) will do a nice job of calculating the BC from those values and then return sight settings, ballistics information, and much more, at any range.

http://www.tmtpages.com/Ballistics_Ver-4_Help/bm88.png

http://www.tmtpages.com/Ballistics_Ver-4_Help/bm94.png

http://www.tmtpages.com/Ballistics_Ver-4_Help/bm39.png

http://www.tmtpages.com/Ballistics_Ver-4_Help/bm43.png

Hope this helps.

[North_of_60]

83281

Great stuff! On my wish list for sure.

[MtGun44]

I was going to say "no".

I stand corrected.

It would be interesting to see how well the calculations correlate with reality. Being a lifelong
computer simulation person, I think this is pretty cool - but it has to be real.

Not like the "climate models".

Bill

[williamwaco]

I was going to say "no".

I stand corrected.

It would be interesting to see how well the calculations correlate with reality. Being a lifelong
computer simulation person, I think this is pretty cool - but it has to be real.

Not like the "climate models".

Bill

Bill - I believe even the best calculations are estimates.

Many of the bullet manufactures state they calculate their BCs in reverse by actual firing tests.

That said,

The Lyman Cast bullet manual lists BCs for all their bullets and many other manufacturers molds are virtual copies of an earlier Lyman.

[mpmarty]

Keep in mind that BC changes with changes in muzzle velocity too. Why worry?

[Larry Gibson]

The Oehler m43 actually does measure the TOF with 2 separate sets of screens; one at the usual 15' to start screen and the other in front of the 100 yard target. From the TOF the BC is computed based on the "G" form selected.

Larry Gibson

[singleshot]

Larry's answer matches my best answer: use 2 chrony's, one in front of the muzzle and one in front of the target. Like any complex system, a simulation is only going to be 50% accurate, but I'm only 50% sure of that. :-)

[North_of_60]

It at least gives a starting point. Aircraft manufacturers and race car designers use aerodynamic computer aided design programs to develope new products so there must be some resemblance to reality in the computers calculations and predictions.

I think that .302 BC in the screen shot I posted above is a little high for that boolit. After I test it at 50, 100, and 150 yards and compare the actual drop with the trajectory that the balistics program on y computer predicts I'll know. Or, ill have something else to ponder.

The bullet design program on Tom's web sit is the one I'd like to play with.

[Tom Myers]

I think that .302 BC in the screen shot I posted above is a little high for that boolit. After I test it at 50, 100, and 150 yards and compare the actual drop with the trajectory that the balistics program on y computer predicts I'll know. Or, ill have something else to ponder.

You are most likely correct in your assumption that the BC is somewhat high for that bullet desing.

The online BC calculator uses a general calculation algorithm that is adapted to give the best fit estimate results from a broad range of bullet designs whereas the Precision Bullet BC Estimator uses more refined algorithms to cover a broader range of design characteristics.

Using your entry values shown below in the software package returns a lower BC that is most likely closer to to what real world test results would show. You will notice that the software package has data entry windows that can record the Advertised BC, Calculated BC, Tested BC and also a Working BC that is to be used in trajectory calculations with whatever ballistic calculation software that is to be used.

http://www.tmtpages.com/LinkSkyImages/forum_images/Bullet%20BC%20Calculator.png

As has been stated, calculated BCs are only an estimate and, at best, a starting or reference point for trajectory calculations.

However, BCs that are obtained through applying actual velocity or drop values to a carefully constructed model (usually the G1 Drag Table) that is similar to the projectile in question can be used to return velocity and drop values associated with varying muzzle velocities. Those values can be quite accurate if bullet stability is maintained and prevailing atmospheric conditions are entered into the calculations.

[North_of_60]

Thanks Tom.

I'm getting close to ordering a mold from Mountain Molds but I am trying to find out what effect a longer or shorter bore riding section would have on the boolits stability and effectiveness as a game stopper. I think that .267 is a respectable BC for a 405 gr CB.


And I still have to pour a cast of my riles's chamber so i can get the nose band the correct diameter and length. I thought I could find sublimed suffer here in Fairbanks but so far I have not. I'll have to order some.

Al

[MtGun44]

William,
OF COURSE you are exactly correct. I have been doing simulations of various types
for my whole career, and sims are only as good as the test data and programming that
go into them. If your sim (calculation, estimate, model, whatever you want to call it)
won't match experimental data, it is junk. If it matches very well, you are golden, if
it matches "reasonably well" it may be useful. Ultimately, the experimental work is
truth and the simulation is an approximation to that, with the quality of the approximation
the key question.

Bill

[BAGTIC]

Considering that almost all BC published by the bullet manufacturers are wildly optimistic an approximation is about all that is available. With lead bullets it must be considered that BC based on form will also be optimistic. It is not unusual for lead bullets to shorten and change the shape of the ogive upon firing. The calculated BC represent a correlation between the bullet being used and the original bullet for which the brag functions were calculated and few lead bullets come anywhere near the shape of the original. Plus the BC varies with speed. The only way to know what your bullet will do in your load from your gun is to shoot it.