Location: The rifle range, or archery range or behind the computer in Alaska

Posts: 3,740

Re: ballistic coefficient on bullets

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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.

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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

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. [img]/ubbthreads/images/graemlins/frown.gif[/img] 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!

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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..

[ 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..

Location: The rifle range, or archery range or behind the computer in Alaska

Posts: 3,740

Re: ballistic coefficient on bullets

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.

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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

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.