ballistic coefficient on bullets

Michael Eichele

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how do I figure actual BC? I have a ballistic program with options for "figure bc from velocity" and " figure bc from trajectory" does it matter which?


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Its up to you, but I prefer shooting out to 1K in 100 yard increments. I note the real atmosheric conditions at the time. Temp, humidity, and raw pressure. Then I document how many hold overs it needed for each bullet drop at each range. I convert them to inches. Now I have real world drops. The problem is, every time the conditions change, so will the trajectory. Thats where software come into play. YOu go to your favoraite software and enter the conditions, velocity. zero ect...Then you play with the drag function and ballistic coefficient untill the computer generates the exact same trajectory as the real world. Then all you have to do is change the conditions in the program and it will give you spot on results. The computer automatically adjusts the BC for the conditions, The BC you enter should remain the same.

I dont like using the 2 velocity method because often times 0-300 yards the bullet is still settling down and will have a lower BC than the average BC over distance. That is however what I prefer. Others have had good results with the 2 velocity method.
 

goodgrouper

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Well, the best way to measure bc is with Doppler radar but unfortunately, most folks don't have access to a multi-million dollar facility. /ubbthreads/images/graemlins/frown.gif So, I use a combination of the two methods already mentioned. I set up two chronos and get a difference in velocity and then plug it into the program and my real bc is calculated. Then i test it at multiple ranges and "backdoor approach" the ballistic program chart until it matches my real world drops. Usually, because of atmospheric conditions and wind being impossible to control or predict, the two chrono method provides about 75% of the trust I have in the number. The shooting makes up the remaining 25% and serves as a "proof" for my initial results. That is, until I can find 1000 yards of wire and run an Oehler 43 acoustic screen out to the target!
 

Mikecr

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'Facts' are genrally defined with math. Is there math out there regarding 'overstabilization'?

In other words, an Sg of >X combined with a drop velocity of Y"/sec, causes a BC decrease of Z%
I haven't seen this from McCoy, Lija, Sierra, or anyone else. Nor is this phenomena accounted for in ANY ballistic software that I'm aware of. I haven't seen a drag curve supporting it.

I just don't see evidence of it. That being, tests, results, and math which defines or predicts it.
Maybe you guys with enough range and equipment available, could prove it. Maybe examples..
 

edge

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[ QUOTE ]
'Facts' are genrally defined with math. Is there math out there regarding 'overstabilization'?

In other words, an Sg of >X combined with a drop velocity of Y"/sec, causes a BC decrease of Z%
I haven't seen this from McCoy, Lija, Sierra, or anyone else. Nor is this phenomena accounted for in ANY ballistic software that I'm aware of. I haven't seen a drag curve supporting it.

I just don't see evidence of it. That being, tests, results, and math which defines or predicts it.
Maybe you guys with enough range and equipment available, could prove it. Maybe examples..


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http://www.nennstiel-ruprecht.de/bullfly/fig15.htm

edge.
 

Michael Eichele

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While Sierra doesnt have any mathematical formulas that I am aware of, they do preach this principal.

I do know this, if it takes a 14 twist to stabilize a 155 30 cal AMAX and you run it through a 1-10x, the BC will always be lower in the 10x vs the 14x unless you drastically reduce its velocity to a level that isnt practical. The differance at 2900 FPS could be an average of about up to .060 of a BC. This is based on real world experiance. Does that make it fact? It is a fact that it happened to me. Do I have any mathematical formulas to calculate and predict what differing bullets and twists will be or know of any? No.

Also, overspining causes slightly more spin drift. Not near as much a BC decay though.
 

Mikecr

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meichele
That's a good example. But have you determined that the BC loss occured near muzzle release, or downrange later?

I realize it wouldn't be easy to prove without serious equipment(like radar, or atleast an Oehler43). And nobody owes any proof. But I am curious about it.
If it's so easy to state as fact, then surely it can be defined. Wouldn't there be a drag curve capturing this?

Maybe someone with a very high twist 22 wildcat, could check drops against another 22 with a more moderate twist. A bullet would have to be used that would work in both, and drops would need to be checked at medium as well as long distance. If this effect occurs, drops would be verified equal at medium distance. But there must be a significant departure in drops at a certain distance and beyond.
 

Michael Eichele

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[ QUOTE ]
I realize it wouldn't be easy to prove without serious equipment(like radar,

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Aint that the truth!!

You bring out some good points and ideas.

My experiance here is somewhat limited and for the most part unscientifec by what most concider to be credible science practices. I certainly dont want to come accross llike the experimentations I have done make anything fact. As has been brought out many times over, there is so much involved in BC that it can make a seasoned shooter's head spin, therefore, what phenomenons I have seen doesnt meen that the reason is what I think it is. It is easy for me to assume that if there is a drastic differance in RPM at similar velocities and there is a corisponding change in BC, its easy to conclude that this must be the reason. Of course, thats not to say that this is responsible for only part of the change.

This is an awesome thread!

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But have you determined that the BC loss occured near muzzle release, or downrange later?

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That is a good question. No I have not ever determined where the loss occurs. /ubbthreads/images/graemlins/smirk.gif I can speculate, but wont as I cannot back any speculation up with hard evidence. That would be interesting to research!
 

goodgrouper

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I'm afraid I don't understand. The math for this is in the texts I quoted. If you don't have access to them, look in the link Edge provided or run a search on Google. Being not a math whizz, I can guarantee you I did not make it up. It looks all like hyroglyphics to me! I just read the english versions and then look for them when doing my own experimentations.
 

goodgrouper

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[ QUOTE ]
While Sierra doesnt have any mathematical formulas that I am aware of, they do preach this principal.


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Absolutely. It's in the first couple chapters if memory serves. Their tests were with a .22 caliber 80 grain matchking fired from barrels with twist ranging from 14" to 7" I believe and the impacts of all the groups and the evidence overstabilization were illustrated in the chapter.
 

goodgrouper

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[ QUOTE ]


Quote:
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But have you determined that the BC loss occured near muzzle release, or downrange later?


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That is a good question. No I have not ever determined where the loss occurs. I can speculate, but wont as I cannot back any speculation up with hard evidence. That would be interesting to research!




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I would assume it can happen very quickly. Look at the 100 yard br crowd again with the 14" twist they run for optimum accuracy. If overstabilization didn't exist or showed up more further downrange, why don't they run 8" twists so they could shoot all manner of bullet weights?
 

goodgrouper

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The first chrono's center must be exactly 300 feet (100 yards of course) from the center of the 2nd chrono. And the first chrono has to be 10 feet from the muzzle (at least according to Oehler's intruction manual).
 
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