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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
BALLISTIC COEFFICIENTS/Twist Rates
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<blockquote data-quote="LouBoyd" data-source="post: 419526" data-attributes="member: 9253"><p>I'm not advocating trying to make a complete drop chart for each bullet/load/rifle. What I'm suggesting is that accuracy can be improved by doing shooting tests at the ranges you expect to shoot a particular rifle, then adjust the ballistics program to give a better match to real trajectories. It does help to have some instrumentation like a chronograph, a method of measuring air density, and to do the test in low/no wind conditions. The BC is a reasonable number to tweak to get the drop to match if you have good certainty of your velocity and air density, particularly since many programs already allow for multiple BC vs velocity inputs. For those programs which calculate spin drift the twist rate input to the program can be tweaked to match the shooting measurements of horizontal drift. Wind can certainly interfere with the tests, and can easily be larger effects than the errors that the shooting tests are attempting to correct. I use manufacturers BC's as a starting point and G7 values when they're available. I'm not trying to write a new ballistics program wich does not use BCs and there would be no available input data if I did. Should shooters attempt to do make these corrections? Only if they have the time and care to go to the effort.</p><p>At lest though they should make some shooting tests just to verify that their computations are close to reality. It's a little late after they shoot an elk in the leg and it hobbles off over a hill 800 yards away. </p><p></p><p>Shooting tests are a way to correct for situations such as the original poster was asking about, in that case why the spin rate appears to affect BCs. Few ballistics programs include the code to predict that effect, whatever the physical explanation may be. For most hunters getting hits close to the point of aim on first and subsequent shots are more of an objective than understanding the physics of how air affects bullets. </p><p></p><p>I agree that shooting though the transonic region is wrought with problems. Existing McCoy based ballistics programs and BCs are nearly useless for predicting transonic performance. The spin rate of the bullet as it goes transonic is rarely known at all and the programs like McGyro don't calculate the decay of the spin rate vs velocity or time. I am not recommending hunting or even target shooting beyond transonic range and velocities. It's interesting though that transonic shooting used to be common for the military. Many WW1 rifles had sights calibrated to 2000 yards or 2 km. They were used for volley fire on massed troops. It is possible to get pretty good accuracy shooting transonic. Few "long range" modern rifles however have spin rates or bullet designs which make it practical.</p><p></p><p>I'm in that 0.1% you mentiond who is interested in trying to understand the physics including shooting subsonic and shooting though the transonic range. I suppose this is the wrong forum to even bring it up. I do think though that every shooter who uses ballistics software should be aware that all programs are limited models and they need to understand what the limitations are. Used outside of those limits they can give very wrong answers. There are many small amplitude ballistic effects with few, if any, available programs even address. No, I don't expect BCs will go away for sports shooting. Both Sierra's multiple G1 BCs and Berger's publishing of G7 BCs are fairly successful methods for improving the predicted trajectories. </p><p></p><p>Bryan, I don't disagree with anything you said in your last post. You support Berger bullets and their objective is to sell bullets and make them useful to the shooting public. For that I think you and Berger are doing an excellent job. Even I don't think there would be a practical advantage in developing new ballistics programs and doing millimeter radar measurements to get drag functions for each projectile. It could only be cost effective if the equipment was used by many bullet manufacturers. </p><p></p><p>You've reminded me that the effort required and money spent would be better directed. Some of it no doubt toward developing a practical instrument to accurately measure the wind along a bullets trajectory. That would give a greater improvement in long range shooting accuracy than making new ballistics models possibly could even if they are perfect.</p></blockquote><p></p>
[QUOTE="LouBoyd, post: 419526, member: 9253"] I'm not advocating trying to make a complete drop chart for each bullet/load/rifle. What I'm suggesting is that accuracy can be improved by doing shooting tests at the ranges you expect to shoot a particular rifle, then adjust the ballistics program to give a better match to real trajectories. It does help to have some instrumentation like a chronograph, a method of measuring air density, and to do the test in low/no wind conditions. The BC is a reasonable number to tweak to get the drop to match if you have good certainty of your velocity and air density, particularly since many programs already allow for multiple BC vs velocity inputs. For those programs which calculate spin drift the twist rate input to the program can be tweaked to match the shooting measurements of horizontal drift. Wind can certainly interfere with the tests, and can easily be larger effects than the errors that the shooting tests are attempting to correct. I use manufacturers BC's as a starting point and G7 values when they're available. I'm not trying to write a new ballistics program wich does not use BCs and there would be no available input data if I did. Should shooters attempt to do make these corrections? Only if they have the time and care to go to the effort. At lest though they should make some shooting tests just to verify that their computations are close to reality. It's a little late after they shoot an elk in the leg and it hobbles off over a hill 800 yards away. Shooting tests are a way to correct for situations such as the original poster was asking about, in that case why the spin rate appears to affect BCs. Few ballistics programs include the code to predict that effect, whatever the physical explanation may be. For most hunters getting hits close to the point of aim on first and subsequent shots are more of an objective than understanding the physics of how air affects bullets. I agree that shooting though the transonic region is wrought with problems. Existing McCoy based ballistics programs and BCs are nearly useless for predicting transonic performance. The spin rate of the bullet as it goes transonic is rarely known at all and the programs like McGyro don't calculate the decay of the spin rate vs velocity or time. I am not recommending hunting or even target shooting beyond transonic range and velocities. It's interesting though that transonic shooting used to be common for the military. Many WW1 rifles had sights calibrated to 2000 yards or 2 km. They were used for volley fire on massed troops. It is possible to get pretty good accuracy shooting transonic. Few "long range" modern rifles however have spin rates or bullet designs which make it practical. I'm in that 0.1% you mentiond who is interested in trying to understand the physics including shooting subsonic and shooting though the transonic range. I suppose this is the wrong forum to even bring it up. I do think though that every shooter who uses ballistics software should be aware that all programs are limited models and they need to understand what the limitations are. Used outside of those limits they can give very wrong answers. There are many small amplitude ballistic effects with few, if any, available programs even address. No, I don't expect BCs will go away for sports shooting. Both Sierra's multiple G1 BCs and Berger's publishing of G7 BCs are fairly successful methods for improving the predicted trajectories. Bryan, I don't disagree with anything you said in your last post. You support Berger bullets and their objective is to sell bullets and make them useful to the shooting public. For that I think you and Berger are doing an excellent job. Even I don't think there would be a practical advantage in developing new ballistics programs and doing millimeter radar measurements to get drag functions for each projectile. It could only be cost effective if the equipment was used by many bullet manufacturers. You've reminded me that the effort required and money spent would be better directed. Some of it no doubt toward developing a practical instrument to accurately measure the wind along a bullets trajectory. That would give a greater improvement in long range shooting accuracy than making new ballistics models possibly could even if they are perfect. [/QUOTE]
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