GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on May 21, 2021, 04:12:44 PM
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A while back I wrote an article for Hard Air Magazine about the new Drag Model for airgun slugs calculated by Miles Morris.... You can find that article here....
https://hardairmagazine.com/ham-columns/some-new-airgun-slug-drag-models-by-miles-morris/
I plotted out the drag coefficients that Miles calculated for airgun slugs with various Meplats, and here is what they look like....
(https://hosting.photobucket.com/images/oo221/rsterne/Miles_Slugs_CDs.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/af9ee85f-aaea-4fcc-98f6-eacd2efb9ffb)
As you can see, the drag is basically the same for slugs having a meplat of 50% of the caliber or smaller at most of the velocities we are interested in.... This coincides with the current trend of swaged slugs available from commercial sources, and if you plot a typical drag model from Miles, along with the commonly used G1 model, and the "corrected" drag for the GA model, you get these results, assuming a constant Sectional Density....
(https://hosting.photobucket.com/images/oo221/rsterne/Miles_Slug_Comparison.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/ccaeddcc-bacd-4234-96a6-f5d70580df64)
Note that the GA curve above doesn't look like what you may be familiar with from Chairgun, because that curve was "adjusted" so that the BC's calculated looked similar to those using the G1 profile, just to avoid confusing us.... ::) .... The important thing is the drastically different shape of the drag curve, extending the point at which the drag rises sharply to about Mach 0.9 instead of Mach 0.8.... This would result in the ability to use about a 100 fps higher muzzle velocity without seeing the increase in wind drift we see now at over 900 fps.... Here is a chart showing the difference....
(https://hosting.photobucket.com/images/oo221/rsterne/Wind_Drift_Miles(1).jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/791a1163-57ea-4463-8c61-c5f145bec8e9)
Miles did not calculate any data beyond Mach 1, as it was unreliable and the trend in the high Subsonic was very clear, with a huge drag increase because of the flat nose, just as you would expect.... Please note that the above wind drift chart is only a "possible" outcome, based on Miles calculated drag models for airgun slugs with various sizes of Meplats.... It will need a lot of testing to confirm it, but it does show that it likely makes sense to try muzzle velocities around 1000 fps when shooting slugs.... We may get a pleasant surprise that we can push closer to Mach 1 without losing out to increased wind drift, like we see with pellets and some slugs....
Bob
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Bit of a rant but, air guns shooting slugs is a sensitive subject for me, so take my opinion with a grain of salt...
I think there are those already racing this horse at the track, the issue is the horse requires a lot of feed (barrel length, pressure, custom mold, full bore porting), and howls like a wounded dog, which at that point, you might as well just use a pb as a propellant.... in reality when you compare the two, is it really any different once you approach the same energy levels @ the muzzle? Semantics sure, until those grey areas are filled...
I think the line between pb and airguns is only becoming thinner. I already have iterations of airguns schematics that can replicate pb power on available design margins (pressure mainly)....but you won't see me bringing it to these forums or market...
For the very, very few this may apply to (and again are already likely racing this horse), I don't mean to offend you whatsoever...
I got into airguns FOR their added safety with projectile decay rate. I'm not trying to shoot 200 or 300 yards with an air gun... If I wanna buck the wind, I'll do so appropriately....I'd rather shoot into the wind, or with it, not against it...
To me air gunning is a discipline, just as any practice in life. I don't think its very disciplined to use air as a propellant in severely windy conditions, nor should a hunter try to dope unpredictably or with such a wide range of shot placement variability...
I'd love to see progression of air gunning, but I do not feel slugs at pre-transonic velocities is the solution for the masses, only for a select few... I would hate to see manufacturers slapping rifles on Wal-mart shelves with these capabilities.... its far reaching now, but normalize slugs and high velocities enough, and we will get there...
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You are, of course, entitled to your own opinion.... Just be aware that airgunning is fortunate in that we operate in the sweet spot for minimum wind drift.... Increasing the velocities to PB velocities just increases the drift (with the same projectile) until you exceed about 2500 fps.... This means (assuming we operate in that sweet spot, shooting slugs) we can have less wind drift than most, if not all, rimfires....
There is no question that a centerfire PB can easily exceed 3000 fps, and likely do so with an even heavier projectile, and hence has the ability to trump anything we can do with air.... In the USA, airguns, federally, are not firearms, so I can understand the reluctance by some to not see them have capabilities that might blur that distinction, and hence gain them attention they don't currently have.... In Canada (and the UK, Australia, New Zealand and many other countries), airguns are already firearms, so if we experiment with improving their capability there is no danger of them suddenly becoming a firearm, because they already are.... ::)
Bob
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What is "Average Meplat"?
I'm currently using Arsenal 257520 BT slugs. 71.5gr at about MACH 0.9 +/-. 1:14 twist. Getting a G1 BC in the 0.16 to 0.17 range out to 50-75yds. They have a 58.3% Meplat.
Would there be a measurable improvement in BC with smaller Meplat? Still searching for a 70gr +/- mold with a <50% Meplat.
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That green curve on the first chart was the average of the 25%, 50% and 75% curves.... It was very close to what Miles got when he calculated the drag for a 65% Meplat.... There is virtually no difference in the calculated drag between the 25% and 50% Meplats, but a significant improvement going from 65% down to 50% (and an even bigger improvement going down from 75% to 65%).... Since most of the airgun slugs seem to be between 50-60%, the red curve I used for comparison with the G1 and GA curves was about what I would expect with a 55% Meplat.... I would think your 58% would be closer to the 50% than the 65% (average) curve, so you likely won't see a lot of improvement at under 50%.... It should about match the red curve I used to compare to the G1 and GA models....
The big thing to take from this is that using the G1 drag model will cause you to find a different BC depending on the velocity you test at.... If your average velocity is between Mach 0.45-0.8 (500-900 fps) the G1 model isn't too bad.... but if you measure your BC at an average velocity of Mach 0.9 (about 1000 fps, where the drag is less than the G1) and then calculate it using the G1 model, you may get a higher value than what you would see at lower velocities.... We see this in the testing for H.A.M., where they are getting lower BC(G1)'s than the manufacturers are claiming because they are testing at under 900 fps.... The manufacturers are typically testing at higher velocities, and then calculating their BC using the G1 model, which gives a higher BC(G1).... As the slug slows down at long range, it likely doesn't live up to the claim....
Bob
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This is outstanding, I’m looking forward to the day I can just select this model in the calculator. Outstanding work and a 1000 thank you’s.
I think we have some interesting capability with the .177 slugs on the horizon. I’ve seen some substantial FPE and accuracy at 100 yards. Obviously I’m just as happy for the .22 and .25’s
I had just mentioned to Cedric I was waiting to hear on these drag models.
If by chance lightening strikes twice I’m also curious to see any further developments in the frangible slugs
*someone needs to write a book and interview Bob
Thanks again to Miles for the models
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.... but if you measure your BC at an average velocity of Mach 0.9 (about 1000 fps, where the drag is less than the G1) and then calculate it using the G1 model, you may get a higher value than what you would see at lower velocities.... We see this in the testing for H.A.M., where they are getting lower BC(G1)'s than the manufacturers are claiming because they are testing at under 900 fps.... The manufacturers are typically testing at higher velocities, and then calculating their BC using the G1 model, which gives a higher BC(G1).... As the slug slows down at long range, it likely doesn't live up to the claim....
Bob
I think that might be what I'm seeing. Highest muzzle velocity tested so far is 975, and that gives the best BC (>0.18) out to 25 yards, but falls off with lower muzzle velocity, or longer distances.
So G1 is not quite right for this projectile. Would a prefect drag model give a relatively constant BC, regardless of velocity?
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Yes, a perfect match between the drag model and your slug would result in a constant BC, regardless of velocity.... That is how the BC is SUPPOSED to work.... ::)
Lapua create a unique drag model for each bullet they make, based on the curve of Cd vs Velocity for THAT bullet.... That is the only way to insure that you can predict the trajectory and wind drift precisely.... The technology is available to do this, but expensive and time consuming.... Miles recounts how the high powered ballistics radars he worked with could track artillery shells for several miles, obtaining exactly this sort of performance data.... 8)
Bob
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The first software I know of which incorporates the drag law can be found here. https://gpc.fotosoft.co.uk/Home.html There are android and PC versions I believe.
Please remember this is a very initial attempt at a suitable drag law which will undoubtedly change as more information becomes available.
The big Doppler radar (made in the USA) I used to use had over a kilowatt of power and could track a shell to 60+km. 50 cal bullets could be tracked to 6+km where they splashed down. The smallest bullet was a 4.6mm which again it tracked to splash at about 2.5km.
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I feel very fortunate that people like Miles and Bob are moving our sports forward 😊 —
and giving us airgunning stuff to do while we're waiting for the weather to clear up, or the pigeons to return, or the back to stop hurting.
THANKS!! We appreciate you!! 👍🏼
Miles,
I'm not familiar with the software you mention. I'll have to check that out! 👍🏼
The cellphone/ tablet apps that I am aware of that permit the user to enter their very own custom drag model (CDM) are:
▪Strelok Pro (frequently updated)
▪Exterior Ballistics (last update 2018)
Manual: https://sites.google.com/site/scixddevexteriorballistics/home/user-manual (https://sites.google.com/site/scixddevexteriorballistics/home/user-manual)
Download:
https://play.google.com/store/apps/details?id=com.ebc.ui&hl=en (https://play.google.com/store/apps/details?id=com.ebc.ui&hl=en)
▪Then of course there is the Windows program ChairGun Pro that permits CDM's, but not their cellphone app.
▪Strelok Pro (Android and Apple) also permits a less involved method toward more accurate use of BCs. It allow you to enter up to five different BCs for any projectile — for five different velocities.
Matthias
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The first software I know of which incorporates the drag law can be found here. https://gpc.fotosoft.co.uk/Home.html There are android and PC versions I believe.
Please remember this is a very initial attempt at a suitable drag law which will undoubtedly change as more information becomes available.
The big Doppler radar (made in the USA) I used to use had over a kilowatt of power and could track a shell to 60+km. 50 cal bullets could be tracked to 6+km where they splashed down. The smallest bullet was a 4.6mm which again it tracked to splash at about 2.5km.
Absolutely fascinating. Thanks again.
I worked in the building the ENIAC was used in. Back then it was called ballistic research laboratory, for my decade of work and currently it’s just army research laboratory.
In 30 seconds the ENIAC completed 20 hours of human computation.
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The ChairGun Custom Drag Profiles only allow input every 0.1 Mach.... That is OK below about Mach 0.8, but from there to Mach 1.2 (ie in the Transonic) I think finer resolution is needed (intervals at Mach 0.95 and 0.975 at the least)....
Bob
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🔵 Miles,
your link to G P Conway's ballistic apps* is very -- no, it's extremely helpful!!
I had no idea his apps existed.And it seems like he's been working with you on this -- as your revised GA2 and slug drag models are also included.
So helpful! Well designed. Even maximum range at angled shots is included, great tool.
Basically a lot of ChairGun's functions -- but now usable on my cell phone in my pocket! And with much better precision than the ChairGun mobile version that had been "dumbed down" from the PC version.
THANK YOU!
*https://gpc.fotosoft.co.uk/Home.html (https://gpc.fotosoft.co.uk/Home.html)
🔵 J.R.,
I just read up on the ENIAC computer. Fascinating!
"ENIAC (Electronic Numerical Integrator and Computer)was THE FIRST programmable, electronic, general-purpose digital computer" in the world. "It was heralded by the press as the 'Giant Brain.'"
https://en.wikipedia.org/wiki/ENIAC
And WHAT was it built to do? Calculating ballistic trajectories!! How cool!! Strelok, ChairGun, GPConway's ballistic apps, they all stand on the shoulders of the ENIAC giant.
So cool,
Matthias
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🔷 And let's not forget the British Colossus that made its debut a year earlier than ENIAC — doing cryptanalysis, cracking Nazi High Command messages and thus helping to end WWII.
"Colossus is thus regarded as the world's first programmable, electronic, digital computer," says https://en.m.wikipedia.org/wiki/Colossus_computer (https://en.m.wikipedia.org/wiki/Colossus_computer)
Matthias
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🔵 Miles,
your link to G P Conway's ballistic apps* is very -- no, it's extremely helpful!!
I had no idea his apps existed.And it seems like he's been working with you on this -- as your revised GA2 and slug drag models are also included.
So helpful! Well designed. Even maximum range at angled shots is included, great tool.
Basically a lot of ChairGun's functions -- but now usable on my cell phone in my pocket! And with much better precision than the ChairGun mobile version that had been "dumbed down" from the PC version.
THANK YOU!
*https://gpc.fotosoft.co.uk/Home.html (https://gpc.fotosoft.co.uk/Home.html)
Which app did you install?
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All of them!! 😄
There are such cool calculations — and all right in my pocket!!
Besides the obvious trajectory calculator to set up for a shot in the field there is:
▪Vertical stringing calc. (e.g., how much will my POI vary up/down if I have a MV extreme spread of 9fps?
▪BC calculations in all imaginable ways:
two velocities and a rangeone velocity and bullet droptime between points and a range
▪Point blank range calculations
▪Trajectory validation
▪Turret click validation
▪Scope magnification validation
▪PCP air use, pressure, shot count estimator, efficiency calcs., etc.
▪Wind calcs.
▪Conversions for airguns
So, who is the developer, GP Conway, anyhow? He doesn't charge for the app, no advertising, no fishy permissions required — it's as if he is a forum member here (or some other AG forum), who just wants to serve the community!! 😊
Matthias
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George is on the UK Airgunforum among others.
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Which app has the BC calculations?....
Bob
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Which app has the BC calculations?....
Bob
That would be the EasyBC app.
And I'll attach a screen shot of the drag models available.
The BC calculator allows for precise atmospheric conditions.
And it allows you to specify errors in the velocity readings of the chronographs, and the range, in order to define the confidence level.
Matthias
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My Norton Anti-Virus will not allow me to download the Windows version of EasyBC.... :(
Bob
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My Norton did the same and removed the setup file but you can tell it to put it back. I have scanned the instaled programs and there were no threats found. i think Norton objects because there have been so few users who have downloaded this file.
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I don't know how to bypass Norton....
Bob
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So if I understood this correctly this model is for flat base slugs that are now produced commercially. Does this model work with rebated boat tail bullets and gas check tails?
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Yes, for flat based slugs with a Meplat of about 50-60%, as are now produced commercially.... It is not intended for use with a rebated boattail, but should be close with a gas-check IMO.... I think if will probably be close for a RBT than using the G1 model, however.... The big difference is that it predicts a lower drag between Mach 0.8-0.95 (900-1070 fps) than the G1 model.... which is consistent with what we are seeing in testing....
Bob
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Thanks to Miles I was able to get EasyBC running on Windows 10.... It is an easy to use program, giving 3 different ways to find your BC, POI change, time of flight and MV, or the more traditional two velocities separated by a known distance (which works with LabRadar too).... It has quite a few drag models which are usable for airgunners, including Miles new proposed Slug model (which he calls SLGo).... On first blush, it would appear to be better for slugs than using G1, so you might want to give it a try.... and don't be afraid to push your slug velocities into the mid 1000s, if they are still accurate for you at that speed.... Don't forget to put in your temperature, barometric pressure (correct for your altitude, and critical), and relative humidity (less important).... If you are shooting at a couple thousand feet and don't use the correct pressure you will get a BC that is much higher than it actually is.... ::)
Bob