Split: BC discussion Pellets
- Thread starter steve123
- Start date
Cole....it’s not easy to get 3 guns all zeroed to the exact same neutral windage unless you have an indoor facility. My successes in FT increased when I began zeroing indoors before a match....because no matter how still the conditions appeared to be outdoors, there was always a difference. I zero my windage at home and the gun travels in an upright position in a cradle with no possible way of anything disturbing the scope.
That could be one factor in your data. Your measurements could easily be skewed 1/8-1/4” either way or more.
Mike
That could be one factor in your data. Your measurements could easily be skewed 1/8-1/4” either way or more.
Mike
Upvote 0
Good points. When using a poly or STX barrel I get about the same BC as JSB has published. But with other barrels the BC is usually less. I think @stoti did some testing a few years back that compared the same pellet from various barrels and measured BC. He came to the same conclusion that the barrels that marked the pellets less had better BCs.
Upvote 0
When LD and I tested some barrels for my USFT, we zero'd in his tunnel and measured the max deflection and also the dispersion. The barrel that was on it was the worst and we tried to see what we could do to save it. It was a standard rifled HW and the best was a poly LW. The above mentioned 13 ft lb rifle was a 12 groove LW from the mid 90s and had the smallest vertical but was close enough in dispersion and max deflection to the poly that we had to call it a tie. This was with 10.3s on the USFT on all barrels running about 900 fps and 8.4s on the 13 ft lb rifle.
Interesting experiment. The bc MAY not tell the whole story but it really should be close. Marks on the pellet or instability from the barrel would show up as increased drag. I guess a lot goes back to measuring equipment and methods. I use a LabRadar but know that EVERYTHING it displays isn't valid. I do try to get enough data to take averages.
From what I've seen, Mike has studied barrel prep immensely and seems to have a decent formula, as his rifles are incredibly good. I'm with Bob Sterne, as I don't really see enough remaining time to REALLY solve this difficult equation. It IS nice to see more data coming out frequently, though.
Bob
Interesting experiment. The bc MAY not tell the whole story but it really should be close. Marks on the pellet or instability from the barrel would show up as increased drag. I guess a lot goes back to measuring equipment and methods. I use a LabRadar but know that EVERYTHING it displays isn't valid. I do try to get enough data to take averages.
From what I've seen, Mike has studied barrel prep immensely and seems to have a decent formula, as his rifles are incredibly good. I'm with Bob Sterne, as I don't really see enough remaining time to REALLY solve this difficult equation. It IS nice to see more data coming out frequently, though.
Bob
Upvote 0
We both know that you know more about these things than myself and most anybody else involved here, but for the purposes of the discussion......remembering that BC = SD / FF and sectional density is SD = M/A, we're basically looking at BC = (weight/cailber)/GA. Weight and caliber are measurable, they have actual units and devices capable of determining them (scales, calipers, micrometers, etc). The form factor is the giant question mark that quite literally is not measurable or quantifiable. The fly in the ointment if you will.
A different degree of marks on the pellet from what we're shooting to whatever data was used to derive the GA form factor could be why we sometimes need to fudge the numbers in our ballistics apps? (ie, we're testing/shooting a very smooth pellet, perhaps fired from a polygonal LW or a FX liner, and the ballistics app is using the GA form factor that was maybe arrived at by a "rough" pellet, perhaps one shot from a HW or LW or BSA 12 land and groove barrel. Round hole in a square peg sort of thing).
Anybody familiar with the origin of the GA form factor? Who developed it? What methods did they use to "figure it out?" Or is it, as Motorhead would say, nothing more than a WAG?
Edit: Just read your comment again, in your view, is "drag" a component of the physics at play that is not accounted for in the BC?
Upvote 0
Some more to add to the discussion, copied and pasted from Gateway to Airguns, from Miles Morris (R Sterne referred to him in that last copy/past I did. I understand he is a ballistician that works with missiles in the UK?).
He added these thoughts after Mr. Sternes comment, that I copy/pasted up the discussion a bit.
"In the world of large calibre ballistics we stopped using BCs sixty odd years ago, as soon as primitive computers could calculate simple trajectories based on using the drag only. The reasons for the move away from BCs was that to achieve the accuracy needed in calculating trajectories you needed a large number of BC values for each speed the shell passed through. As a result the trials that had been used to obtain the BCs were used instead to produce design specific drag laws. Recently the small arms industry has made the miraculous discovery that they can improve long range predictions by using drag laws rather than BCs. Having dealt with the small arms industry I am not surprised it took them about fifty years to catch up.
Unfortunately the airgun ammunition industry seems to still be clinging to BCs. To be fair to both the small arms and the airgun ammunition industries part of the reason for not changing probably lies in the acceptance of anything different by shooters, many of whom seem to be conservative in their approach to anything new. With airguns, certainly in the past, ranges have been relatively small so the short comings of BCs did not make a huge difference in the calculations. As ranges grow however then different approaches will be needed and even the use of a purpose drag law may not be sufficient.
It is perfectly possible that two projectiles with the same BC can behave differently and have different wind responces. This is due to the reference drag law being for a projectile of a different shape to the ones being fired and hence introducing errors in the calculations.
The use of small radars enables drag laws to be produced just as easily as BCs. It requires a slightly different approach but it is no more difficult and the resulting data should give better predictions at long ranges. The biggest source of errors will be poor experimental technique and inadequate instrumentation which also affect measured BCs. Just assume you have to measure everything and you won't be far wrong. However again, at many airgun ranges, the errors from performing a rough basic ballistic test will not be that great.
As for Reynolds number, yes it is something to be aware of, but don't over estimate its importance. I have only once come across trial results which showed a significant reynolds number effect. That was on a small (14.5mm) calibre projectile doing around 300 ft/sec. You also do not always want smooth laminar flow, sometimes turbulent flow will give a lower drag. The problem with normal sized pellets (.25 and below) is that they have Reynolds numbers of a very awkward value.
There is a great deal more which can be said about BCs, drag laws, Reynolds numbers, prediction methods, aerodynamic coefficients etc. but it would probably be more information than most shooters want or need."
He added these thoughts after Mr. Sternes comment, that I copy/pasted up the discussion a bit.
"In the world of large calibre ballistics we stopped using BCs sixty odd years ago, as soon as primitive computers could calculate simple trajectories based on using the drag only. The reasons for the move away from BCs was that to achieve the accuracy needed in calculating trajectories you needed a large number of BC values for each speed the shell passed through. As a result the trials that had been used to obtain the BCs were used instead to produce design specific drag laws. Recently the small arms industry has made the miraculous discovery that they can improve long range predictions by using drag laws rather than BCs. Having dealt with the small arms industry I am not surprised it took them about fifty years to catch up.
Unfortunately the airgun ammunition industry seems to still be clinging to BCs. To be fair to both the small arms and the airgun ammunition industries part of the reason for not changing probably lies in the acceptance of anything different by shooters, many of whom seem to be conservative in their approach to anything new. With airguns, certainly in the past, ranges have been relatively small so the short comings of BCs did not make a huge difference in the calculations. As ranges grow however then different approaches will be needed and even the use of a purpose drag law may not be sufficient.
It is perfectly possible that two projectiles with the same BC can behave differently and have different wind responces. This is due to the reference drag law being for a projectile of a different shape to the ones being fired and hence introducing errors in the calculations.
The use of small radars enables drag laws to be produced just as easily as BCs. It requires a slightly different approach but it is no more difficult and the resulting data should give better predictions at long ranges. The biggest source of errors will be poor experimental technique and inadequate instrumentation which also affect measured BCs. Just assume you have to measure everything and you won't be far wrong. However again, at many airgun ranges, the errors from performing a rough basic ballistic test will not be that great.
As for Reynolds number, yes it is something to be aware of, but don't over estimate its importance. I have only once come across trial results which showed a significant reynolds number effect. That was on a small (14.5mm) calibre projectile doing around 300 ft/sec. You also do not always want smooth laminar flow, sometimes turbulent flow will give a lower drag. The problem with normal sized pellets (.25 and below) is that they have Reynolds numbers of a very awkward value.
There is a great deal more which can be said about BCs, drag laws, Reynolds numbers, prediction methods, aerodynamic coefficients etc. but it would probably be more information than most shooters want or need."
Upvote 0
For my understanding ... (correct me if wrong), the bc is a computational expression of drag where we are trying to predict drag from the components we are able to acquire of the projectile. LabRadar only measures drag.... 5 selectable points to sample the velocity decay. This must be converted to bc to use in the ballistic programs.... which I really like using.
Miles really does have a lot of interesting info... as does Bob Sterne.
Bob
Miles really does have a lot of interesting info... as does Bob Sterne.
Bob
Upvote 0
That's how I view BC too.
Technical conversations with you are very often enlightening and I was really hoping that you have some some crazy deep understanding of what's going on with an airgun projectile as it flies downrange.
And yeah, to get something useful out of a ballistics app, there's got to be some sort of numerical value to account for the velocity decay.
Technical conversations with you are very often enlightening and I was really hoping that you have some some crazy deep understanding of what's going on with an airgun projectile as it flies downrange.
And yeah, to get something useful out of a ballistics app, there's got to be some sort of numerical value to account for the velocity decay.
Upvote 0