finding swaged bullets BC

[phishroy]

Well, not exactly swaging but swaging related.

I have been starting to become more serous lately with some long range precision shooting.
I would like to use some of my own swaged bullets for it but Iwas wondering if anyone has any tips on how to figure out a bullets ballistic coefficient.
The bullets BC would greatly help in generating some of the ballistic charts.

ive seen on the JBM website a method to calculate it but I would need 2 chronographs to do it, which I do not own.
Was just wondering if any of you out there that swage their own have some tips on figuring out a bullets BC.

[IllinoisCoyoteHunter]

I guess you could do some trial and error calculating...

[Utah Shooter]

If memory serves me right Bullet maker 57 has a way to do it.

[runfiverun]

I think you can calculate it by bullet drop over distance.
you could compare those numbers to a known similar shaped bullets B.C.
if your 165's drop is similar to a sierra 165 target bullet [from 1 to 300] and you have the same ogive shape your B.C. will be super close.

[Smokin7mm]

I came pretty close using JBM and shooting my loads at various ranges and recording the MOA of adjustment and then played around in JBM plugging in the knows (length, dia, weight, velocity, etc) and changing the BC until the drops matched.

Bret

[aaronraad]

I use Litz's G7 BC Estimation formula. It's empirical but it gets me in the ballpark with a nominal +/-5% error as stated by Litz.

I ran the formula against 27 samples of his G7 BC test results from his 2nd edition of Applied Ballistics For Long Range Shooting and got an:

-average error of -1.2%; with
-a Min of -11%; and
-Max of 3.6%; and
-1 standard deviation of 0.0313.

[phishroy]

I use Litz's G7 BC Estimation formula. It's empirical but it gets me in the ballpark with a nominal +/-5% error as stated by Litz.

I ran the formula against 27 samples of his G7 BC test results from his 2nd edition of Applied Ballistics For Long Range Shooting and got an:

-average error of -1.2%; with
-a Min of -11%; and
-Max of 3.6%; and
-1 standard deviation of 0.0313.

can you elaborate. what is this formula or where can i find information about it?
Thank you

[aaronraad]

can you elaborate. what is this formula or where can i find information about it?
Thank you

I can't find a published copy of Equation 19.1: G7 form factor prediction but I'm sure you will be able to source a copy of the Bryan's book locally.

I'd elaborate, but as time goes on and others become involved, profits get squeezed etc. companies have a tendency to become more litigious.

[BT Sniper]

Did you find the "drag and twist" calculations section on the JBM site? I have used it quite a bit, just be sure you enter the data correctly. Most of the data is entered in percentages of the caliber, example...... a 22 cal bullet that is .700 in length will have a total length of 3.125 calibers (.700 divided by .224 equals 3.125)

Check it out

http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi

You should be able to measure your bullets and enter the data to give you a pretty close estimate of BC

Good shooting and swage on!

BT

[onomrbil]

Two chronographs and some math. Drag coefficient F is calculated by: F=(rVavg)/dV where Vavg is calculated by (V1+V2)/2 dV=V1-V2 r=distance in feet V1=Muzzle Velocity V2=Target velocity.

Set up two chronographs, one to measure muzzle velocity, the other at the target stand to get target velocity. Distance (r) is between the two chronographs in FEET. This eliminates guesswork and approximations. The more data you have the better your number will be.

Plug the drag factor into your favorite ballistics software and you can get a BC, but the drag factor is a better datum for calculations . . .

See Arthur Pejsa's book, NEW EXACT SMALL ARMS BALLISTICS for more detailed calculations.

[phishroy]

Hemmm, so looks like i may need to get another chronograph....

[Bill G]

http://www.geoffrey-kolbe.com/drag.htm

This may be of interest to you.
Also, there will be a new chronograph coming out very soon. It is called labdar and samples bullet speed 2X/yrd out to 100 yrds, depending on the bullet size. It is a doppler radar system that is quite affordable and does away with the need for multiple chronys. it is pricey but not really when you consider the functionality. a ggod chrony will cost you 1/2 what this thing is supposed to run, so your right on the mark when you consider buying 2. Oh.... it stands right beside you so never a risk of shooting it.

Bill

[goblism]

http://www.geoffrey-kolbe.com/drag.htm

This may be of interest to you.
Also, there will be a new chronograph coming out very soon. It is called labdar and samples bullet speed 2X/yrd out to 100 yrds, depending on the bullet size. It is a doppler radar system that is quite affordable and does away with the need for multiple chronys. it is pricey but not really when you consider the functionality. a ggod chrony will cost you 1/2 what this thing is supposed to run, so your right on the mark when you consider buying 2. Oh.... it stands right beside you so never a risk of shooting it.

Bill

I punched in some commercial bullets data on this and it was quite close.

[phishroy]

http://www.geoffrey-kolbe.com/drag.htm

This may be of interest to you.
Also, there will be a new chronograph coming out very soon. It is called labdar and samples bullet speed 2X/yrd out to 100 yrds, depending on the bullet size. It is a doppler radar system that is quite affordable and does away with the need for multiple chronys. it is pricey but not really when you consider the functionality. a ggod chrony will cost you 1/2 what this thing is supposed to run, so your right on the mark when you consider buying 2. Oh.... it stands right beside you so never a risk of shooting it.

Bill

Thank you, this is extremely helpful.