BC & wind drift

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  • Last Post 21 September 2020
shootcast posted this 18 September 2020

Two bullets the same BC and same muzzle velocity but different diameter and weight. Same cross wind speed and angle . The bullets arrive at the target at different velocities due to Bullet weight.  The larger diameter Bullet also must endure more side air resistance because of diameter or circumference . The wind has more to push against. So why are 30 cal. Cast bullets still king of the game.

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RicinYakima posted this 18 September 2020

A 220 grain 30 caliber bullet is about all the recoil most old men can stand for a 200 round match weekend. Otherwise we would all be shooting 550 grain 45/70's.

Wind drift is calculated by loss of speed from muzzle to target. Long for caliber bullets loose less velocity. BC is a minor consideration out to 200 yards.

There are 20 good .30 caliber bullet designs for any one of other calibers.

IMHO

 

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joeb33050 posted this 18 September 2020

Two bullets the same BC and same muzzle velocity but different diameter and weight. Same cross wind speed and angle . The bullets arrive at the target at different velocities due to Bullet weight.  

No! Same BC, same MV = same  velocity at all ranges.

 

The larger diameter Bullet also must endure more side air resistance because of diameter or circumference . The wind has more to push against.

Same BC, same MV = same wind drift at all ranges!

 

So why are 30 cal. Cast bullets still king of the game.

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Larry Gibson posted this 18 September 2020

Joe is correct, the bullets with the same BC at the same velocity would arrive on target with the same velocity regardless of weight difference.

LMG

Concealment is not cover.........

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John Alexander posted this 18 September 2020

The concept that wind drift increases as the length or diameter of a bullet increases because that gives the wind more surface to push against is one of  those ideas that seems to be just "common sense" or "clearly logical", but in fact is opposite of the truth.

Between a short and a long bullet with the same nose shape the long bullet will drift less not more, in spite of presenting more surface to the wind.  That is because it will have a higher sectional density which is directly related to BC which is what counts.

if a small caliber bullet is a scale model of a larger caliber bullet the larger bullet will have a higher BC and drift less in spite of presenting more surface to the wind, as shooters using 22 cast bullets should know.

The study of ballistics is complicated. Nature is almost always complicated. The admirable human tendency to try to reason things out logically often goes off the rails because of faulty assumptions or factors not considered. My favorite structural engineering professor liked to say, "If one of the assumptions is wrong, no matter how carefully and precisely the calculations are done, the bridge may wind up in the river -- as engineers find out from time to time." 

In the case of cast bullet shooters, faulty assumptions sometimes lead to false conclusions such as accuracy can be improved by avoiding such things  as variation in neck tension, tiny but visible bullet defects, tiny variation in bullet weights, small variation in case weights, and many others -- all of which seem "logical".

None of these have been proven to be true by experiments. If it can't be shown by careful experimentation, it probably isn't true, no matter how much it seems like "common sense" or " completely logical."

John

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RicinYakima posted this 18 September 2020

Lyman lists their best .22 bullet with a BC of .116 (225415). The best .30 is .377 (311299). Best .45 is .391 (457125).

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Bud Hyett posted this 19 September 2020

There is a way to get an estimated ballistic coefficient using the NOE mold diagrams. NOE offers bullets that are copies of many popular bullets from other manufacturers. If you wish to spend the time, you may find your bullet on their website; then go to the dimensional diagram and find the ballistic coefficient. I've done this and found the data holds up at 200 yards within  an inch or two.

They have an extensive explanation of ballistic factors on the following webpage. 

https://noebulletmolds.com/site/using-our-products/faqs

The explanation also explains the G1 through G8 factors that are essential to an understanding of the term "ballistic coefficient." Unless you are using the same form factor and G* constant, the comparison values are way off. 

I've used this comparison shopping for a bullet in a new caliber, such as the initial search for the .25-20 WCF 120 grain bullet.

While this is interesting and informative, the time spent in speculation is better spent in loading and testing at the range. Once you have an idea of the trajectory, confirm it by firing the load. 

Country boy from Illinois, living in the magical Pacific Northwest

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shootcast posted this 20 September 2020

Several days and several good replies. Thank You. I just want to add some thoughts. I have used the Hornady on line BC Data. Pretty neat by the way. Bullets of the same BC but of different diameter and weight  fired at the same MV show the same drop and wind deflection. Also this from the third edition of the Lyman Cast Bullet Handbook.

Lyman #245496 83 Gr. BC .202 MV 1900 @200 1296 Time of Flight .386 Wind deflection 5 mph 6.18

Lyman # 457191 293 Gr. BC .201 MV 1900 @200 1323 Time Of Flight .382 Wind deflection 5 mph 5.84

The higher the BC the less the air drag ? OK these are not identical.  But close and the 83 should be the better. Very likely I misunderstand the information. Could it be we put to much into this or maybe there just hasn’t been enough test to prove the theory.

 

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alphabrass posted this 20 September 2020

Models are imperfect.  As one of my professors said: "all models are wrong and some models are useful."  There are many assumptions in these models such as the form factor and consistent atmoshphere, both vary during during the bullets flight.  Absent from the models output is the measure of uncertainty in the values.  If one was to test and measure the TOF for two bullets of the same BC several times there would likely be as much variation for one bullet as the difference between the two.  A T-test would give the likelihood that there was a real difference between the two.  Often the model output values are given with superfluous digits and give an impression of certainty that is not warranted.

All of this makes for an interesting pursuit.  Another one of my professors sayings was that a good engineer needs to be lazy.  That is to say always looking for an easier way to accomplish a task.  The pitfall is that one can spend way more time and energy working to develop the "easier" way than just to do it.

  

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Larry Gibson posted this 20 September 2020

alphabrass makes some excellent points.  I can measure the time of flight of the bullet with the Oehler M43 PBL with muzzle screens and screens in front of the target out to 100 yards.  The program then converts the TOF to the BC depending on which "G" factor is input.  Interestingly, by measuring the actual TOF/BC of each fired bullet the stability can also be determined as the more consistent the TOF/BC is shot to shot them more stable the bullet is at launch.  The stability differences between different twist barrels given the same bullet and load is also easily seen.  As to the comparison of BCs Dr. Oehler told me the 3rd digit of a BC is as useful as pondering whether a deer is 173 or 175 yards away.....

BTW; the computer program/models for determining the BC are most often overly optimistic and seldom correct.  Sometimes they are close but that is rare as there are many variables that effect the actual BC.

LMG

Concealment is not cover.........

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Ken Campbell Iowa posted this 20 September 2020

...also the BC of a bullet is not a constant ...  it goes higher or lower depending on instant conditions .

" it is complicated " ...

but the good news is that the published BC values of bullets give us a working idea of what to expect.

i actually kind of like that ...  it helps me to understand people ... their BC varies quite a bit also ...

ken

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John Alexander posted this 20 September 2020

With very few exceptions (precision surveying or machining,  hitting the moon with a rocket, etc.) the third digit of any factor, coefficient, force, etc. relating to the material world is usually irrelevant as well as probably being wrong anyway. Most of the digits, past two, that look so solid and precise on your electronic device seldom have much relationship to reality.

Hardy Cross, a famous engineer of a few generations back is supposed to have told his students that if they replaced the third digit in all their calculations with a "7" it wouldn't make any difference to the structures they were designing.  Yet we pretend to measure to the ragged edge of a bullet hole in paper to the thousandth.  Oh well.

John

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45 2.1 posted this 20 September 2020

The above is a good example of a certain principle. That principle is known as the " Good Enough" principle which is OK with folks who don't carry enough decimals out past the point where rounding errors produce change orders where someone pays for a mistake which can be quite costly. It is also a good example why very few people are shooting somewhat under an inch repeatedly. Sorry for the rant, but I saw this happen repeatedly throughout a long career.

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RicinYakima posted this 20 September 2020

45 2.1 makes a very good point, in that "good enough" is usually good enough for most things. In my first professional life "good enough" was bad, the fire is out or it is not. Having 99.9999% of the fire out is not good enough. In my second professional life, parts per billion of trichloroethylene in water didn't matter. Just because you can measure it, doesn't mean it is important.

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Wineman posted this 20 September 2020

Hitting the moon, or just about anything built prior to that used the humble and almost unbreakable slide rule. Estimate the third decimal place. I just missed the SR generation but pull one out and pretend I know what I could have done with it. A local High School teacher uses them to get students to think about where the decimal point is that so often gets lost on a calculator.

 

Dave

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alphabrass posted this 21 September 2020

Very interesting discussion.  In any random process, like the flight of a projectile, there is uncertainty in a prediction.  Variation in the factors that affect the outcome will result in variation of the outcome.  This is related to, but somewhat different, than the significant digits issue.  There is a band of uncertainty around the prediction, often given as a gaussian curve, aka normal distribution.  One way this is expressed is to say that the velocity predicted will be within plus or minus X% of the time with a confidence level such as 90%.

Letting the inputs vary, say BC and Vo for example, the predicted TOF will also vary.  Varying of the inputs randomly over a number of iterations will generate a distribution for the output.  While I have not addressed some pesky details this is the Monte Carlo simulation used to model random processes.

Knowing what the uncertainty is will inform the significant digits decision.  Understanding and accounting for the uncertainty informs design decisions more reliably than assuming everything is certain.

alphabrass: the 70/30 solution

 

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John Alexander posted this 21 September 2020

My post on being skeptical about any figures beyond the first two of any measure, or especially figures of any estimate, wasn't really about "Good Enough" and especially it wasn't about the type of good enough that tries to justify sloppy work that should be done better as 45 2.1 seems to imply.

"Don't Kid Your Self" describes my point better.  As in don't kid yourself that most of that six digit string of numbers  on your computer screen mean anything at all -- because they usually don't.

Not understanding this is the reason that someone that sorts their bullets to vary less than 1 per 2000, as many do, often gets their butt kicked by a shooter that only sorts to 1 per 200.

John

 

 

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RicinYakima posted this 21 September 2020

Shooting is still an art and not a science. Science will get you a precise load for that weapon. A person still has to shoot it.

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45 2.1 posted this 21 September 2020

Art or Science? Let’s look at this thru the last 150 years of shooting. Ever since rifles were made there has been a drive to make them more accurate. Government arsenals all over the world this as well as individuals. Whitworth did a scientific series of tests to determine what twist shot the best for the English government. Our own Government developed the Trapdoor rifles to a state where it took the bench rest shooters in the 50’s to match what they did. The Schuetzen era developed target shooting to an art, but the shooters could take lessons from past efforts from those still shooting the same way. All this is a series of investigations to develop more accurate rifles and methods. There are reasons why accuracy happens…. It is not a trick (as called by some of our own) or happenstance. The science part is knowing why something happens and the reasons for it. Unfortunately, the Art and Trick predominate with our current shooters. Until you understand what happens and why, you are a part of that culture. So, Art or Science…. Pick your own category where you want to be, but mine is Science.

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John Alexander posted this 21 September 2020

45 2.1

Excellent. You have just described the primary reason that the CBA was started.  The first of our four goals, found on the back of every Fouling Shot, says what your post says in many fewer words. I couldn't agree with you more.

With a journal to share their findings and a competitive program to spur development, members working to achieve that goal have brought the precision of cast bullets in fixed ammunition from where a single one inch group of five shots with a heavy, custom built, target rifle was something to brag about to where aggregates of under 0.5 inch are often needed to win in local CBA matches.

The reason for this improvement was that members were willing to experiment with different bullet designs, loads, and the rifles to shoot them. They were also willing to enter matches where others could verify their groups and scores.

Equally important, they were generous in sharing what they had learned by taking the time to write articles for the Fouling Shot. Their match performance gave them the creditability to inspire others to duplicate their methods and do experiments of their own. This is an example of the scientific method in action.

Unfortunately, we have fewer such experimenters and writers than in the 1900s and progress has slowed or maybe stopped.

CBA members aren't the only people shooting CBs. It could be that there are CB shooters somewhere learning new techniques for shooting  CBs better and keeping the knowledge to themselves, either because they are secretive, or because they would rather kibitz from the sidelines to show their superiority with unverified claims. These folks, no matter how capable, make no contribution to the precision of CB shooting. If there are such shooters, it is a pity because the secrets they hint at with vague clams and jargon go to the grave with them.

John

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RicinYakima posted this 21 September 2020

Not disagreeing with John or 45 2.1's thoughts and posts. However, one person making the best ammunition for one rifle does not necessarily translate into that ammunition being the most precise in another rifle of the same make and model. The art of handloading is identifying and applying those factors discovered by others to exactly your situation. Then applying skill to fire the ammunition and rifle to meet your goal, i.e. small groups.

Many are hoping for the plastic wrapped (powder coated) bullet to be a break through, but no one has stepped up the search for better materials and techniques. Small variations of what we have been doing for the last twenty years has not advanced the science part very well, but there are many more very good shooters than there was twenty years ago.

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