GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on January 21, 2019, 07:31:53 PM
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For a while now I have been working on compiling a database for cast bullets and pellets commonly used in airguns…. It is by no means a comprehensive list, but it represents bullets I have moulds for.... plus those for my Bob's Boattail designs which are available through Accurate, LBT or NOE.... and also the NOE Target, Hunter and Magnum Hunter Pellets I have designed for them.... The Database at the link below is in a .pdf file, printed from my Excel spreadsheet that I use to store the data and manipulate it.... I calculated the Sectional Density, as it is one of the two critical parts of the Ballistics Coefficient, the other being the Form Factor.... They are related by BC = SD / FF.... and I used the G1 drag model throughout this spreadsheet....
The BC's in the column in red are those advertised by the mould manufacturers, and IMO you can take them with a grain of salt.... some are good, some not even close (likely because they were taken at the wrong velocity for our purposes)…. The really bad ones I have shown with a strikethrough so that you will disregard them.... For every bullet in the database, however, I have input their dimensions into the Kolbe Drag Calculator.... http://www.geoffrey-kolbe.com/drag.htm
I then recorded the resulting BC (G1) at Mach 0.5 and Mach 1.0.... For the most part, the BC in the "BC M.5" column will be more accurate for airgun use, particularly if it agrees closely to the manufacturer's claim.... However, I have yet to see a boattail bullet reach those calculated numbers, although they do have less drag Subsonic than the same bullet with a flat base.... Likewise, weights in red are from the manufacturer, while those in black are what my cast bullets weigh.... I typically use 40:1 alloy from Rotometals (2.5% Tin), but some of the weights may be pure Lead or 1% Tin.... Bullets over 100 gr. or above .35 cal. are rounded/weighed to the closest grain....
At the far right is a column labelled FF, which is MY ESTIMATE based on the type of bullet or pellet, the few BC measurements I have made to date, and those from others that I trust.... Those with a pale yellow background are for pellets, and I have only used two values.... For wadcutters I used FF = 3.00 and for the RF nose of the NOE pellets I used FF = 1.67.... That is in the ballpark for similar pellet shapes, and in agreement with NOE's published BC's for their pellets.... For flat based bullets I used FFs between 0.9-1.2, with longer thinner bullets having a lower FF.... and I did the same thing for boattails, with FFs between 0.7-1.1 (pale green background).... I will refine those Form Factors as data permits, but for now they should be reasonably close.... I then used the FFs in that column, and the SD, to calculate my "Estimated BC".... With a lack of hard data, that is the number I would use for an approximate BC at airgun velocities....
You will note two blue columns for the actual BC @ specified velocity at ICAO conditions.... These are gradually filling up with measurements I have input so far, and I hope to continue to flesh out that data over the years.... and yes, that is the expected length of this project.... When/if I have multiple data points for the BC, taken at different velocities, I will pick the one I judge to be the most reliable and/or the closest to 900 fps.... I consider that to be a typical average midrange velocity typically used by airgunners…. as we generally strive for a bit higher muzzle velocity than that in the hope the average velocity through to the target is close to that value.... particularly with bullets, which is the primary focus of this database....
I also ran all the bullets through the Kolbe Twist Calculator.... http://www.geoffrey-kolbe.com/barrel_twist.htm
This is the only twist rate calculator I have found that does a proper job of assessing the twist required in the Subsonic and Transonic range.... Just as the Kolbe drag calculator probably underestimates the drag on boattail bullets, the twist calculator probably tells you they should spin faster than is really necessary.... but it's the best data I have, so that is what I have used.... The number shown is the recommended twist rate, in inches, to give a Stability Factor of 1.5 at a velocity of 1000 fps.... That should maintain stability under any conditions, even at the worst case of Mach 1.... without spinning the bullet too fast as it slows down.... For the larger calibers, where the twist rate is over 20", I have rounded the recommended twist rate to the nearest inch.... Note, it is always better to spin a bullet a bit faster than these values, rather than slower, with the possible exception of boattails as noted above.... There is no such data for pellets, as the Kolbe calculators won't handle Diabolo (waisted) designs.... so that area on the spreadsheet is blanked out for pellets, both for BC and twist....
I hope you find the attached information useful.... It is a .pdf file, set up to print in "Landscape" format, and currently runs 7 pages in length.... If any of you have BC data you wish to share, that is corrected to zero altitude and "standard" ICAO atmospheric conditions, please send it to me via PM and I will consider including it.... In reality, my shooting time to allow me to work my way through all these bullets and pellets is very limited for the next 2 years until we close the Motel in Feb. 2021.... By that time, however, I plan to have a LabRadar doppler radar Chronograph, and the time to use it.... In the meantime, I will use available data to fine tune the Form Factors I am using to estimate BCs and possibly expand the database as well....
Bob
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Here is the G1 Drag Model, showing how the Drag Coefficient increases with velocity....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/G1%20Drag%20Profile_zps7o5uthoz.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/G1%20Drag%20Profile_zps7o5uthoz.png.html)
Here is the exact same chart, with the Cd plotted against the Mach number instead of fps....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/G1%20Drag%20vs%20Mach_zps4xauawqq.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/G1%20Drag%20vs%20Mach_zps4xauawqq.png.html)
The range between Mach 0.8-1.2 is called the "Transonic" range.... Note the extreme increase in drag in that range (over 5X).... This means that bullets launched in that range (or just above it) don't stay supersonic very long.... and it also means that in that range they are at the mercy of the wind and drift more, even though they are going faster.... Add that to the inability of PCPs to achieve Supersonic velocities with any level of efficiency.... and you will quickly understand why staying under Mach 0.9 (1000 fps) is of critical importance to us....
Bob
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Nice work Bob . One question ........ do you ever sleep ?
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Bob, here's more work for you. Give us the blue prints or the dimension drawings. 😁
Thank you in advance!
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You can look up all the dimensions for the Accurate, Arsenal and NOE designs on their websites.... Lee does not do that, so I had to measure a bullet and THEN input all the necessary dimensions directly into the Kolbe Calculator....
and yes, it takes a loonnngggg time to put in that many bullets, one at a time, into the TWO Kolbe calculators, as they don't share the inputs.... ::)
Bob
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Very cool. Nice to see the lee 340-f that I shoot in my texan in there. What lead alloy was used for it?
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Probably pure lead or 1% tin.... I now use 40:1 but that was a while ago....
Bob
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I have developed a list of calculated BCs for pure lead roundball…. It contains the approximate Subsonic BC (G1)…. and also the BC (GS), which should be valid for all velocities below Mach 2, including Subsonic and Transonic, for spheres less than 1" in diameter.... The fact that these two BCs are not even close to the same value shows how important it is to use the proper drag model for the shape of the projectile.... Here is my calculated data....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Airgun%20Roundball%20Database_zpsmiftzuli.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Airgun%20Roundball%20Database_zpsmiftzuli.png.html)
Here are the drag models the above is based on.... Note that the Cd of a sphere is much greater than that of the G1 standard bullet....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20Coefficients%20Below%202000%20fps_zpsq9mxrmkw.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20Coefficients%20Below%202000%20fps_zpsq9mxrmkw.jpg.html)
Just as the Cd at various velocities of the G1 (and G7) drag models has been carefully studied and recorded, the GS drag model was developed by measuring the drag of a standard sphere of 9/16" diameter at velocities up to Mach 2.... If you Google the drag coefficient of a sphere, you will find the value 0.47, which is true at low velocities.... At 1500 fps and above, it increases to 1.00, as shown above.... ChairGun assigns a BC of 0.200 to the standard 0.562" sphere when made of lead and weighing 266 gr.... I have not been able to find that BC assignment in the literature, but Strelok uses the same.... so you can easily calculate the BC (GS) by using BC (cal.) = 0.200 x (cal.) / 0.562.... This is because the SD is proportional to the caliber, and BC is proportional to SD....
If you have the GS drag model available, as in ChairGun or Strelok, you should use that, and then input the BC appropriate for your caliber from the BC (GS) column in the above table.... If your trajectory program does not support the GS model, you can get an approximation of the trajectory by selecting the G1 drag model and using the BC (G1) shown in red above.... I determined by simple comparison in trajectory (to the GS model) over 200 yards using ChairGun (with a MV of 900 fps) that for a sphere, you can calculate the BC (G1) by using BC = SD / 1.60.... ie the FF (G1) of a sphere at 900 fps is 1.60.... NOTE, YOU CANNOT USE THE G1 BC WITH THE GS DRAG MODEL, OR VICE VERSA....
Bob
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Thanks for the great data Bob. So, would the boattails that Nick sells be considered spherical(GS drag function) or the universal G1?
Peter
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G1, all the BCs in the database are G1.... We should probably be using G7 for boattails, but nobody does in airgunning…. Below 900 fps there is little difference in the shape of the drag curve, but you would have to use a G7 BC if you used the G7 drag model....
Bob
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G1, all the BCs in the database are G1.... We should probably be using G7 for boattails, but nobody does in airgunning…. Below 900 fps there is little difference in the shape of the drag curve, but you would have to use a G7 BC if you used the G7 drag model....
Bob
Ok. Thanks for clarifying that Bob. Much appreciated.
Peter
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Database in OP updated, a couple of small errors corrected, .25 cal, .30 cal, and .36 cal buckshot added....
Bob
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Database in OP updated, now at Ver.3.... I made a small revision in the system I have for estimating the FF, starting from a base value and then adding/subtracting credits for the combination of the L/D ratio and the Meplat diameter, or for a boattail.... Basically I am categorizing bullets by their "shape".... For the most part my predicted BCs are now closer to calculated BC M0.5's.... This is a work in progress, and will be revised as I get more data and better ideas.... It is a "visual" system to a large degree, and it will be interesting to see how the numbers compare to reality....
Bob
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Wow! My head hurts! Whew!!! ;D
Thanks a Million Mr. Bob! ;)
Mike
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Database in OP updated again to Ver.4.... Weights added for many of the NOE Pellets, and a couple of other minor changes....
Bob
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I spent another hour looking at the list. And I've been up all night in the shop! LOL!!! ;D
Once again, thank you Mr. Bob for all your hard work and efforts!!!
Mike
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Database in OP updated again to Ver.5.... Weights added for more of the NOE Pellets, and a couple of other minor changes....
Bob
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Well done Bob . I spent time adding your info to my cast bullet catalog . I noticed that the 257420 bullet for example recommends a 1:16 or so twist which is a bit slower than the standard 1:14 everyone is using. Is that due to the drag model?
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Nope, that's just what it works out to for that length and shape.... The numbers I have quoted are to give an SF = 1.3 at 1000 fps (which is adequate), and for the 257420 that is a 16.1" twist.... Consider that the slowest recommended twist rate.... If you checked it with a 14" twist, you would probably find it would be close to 1.5 at Mach 1, which is optimum if you are shooting that fast.... Most guys are interested in the slowest twist that should work, and having a bit more stability is better than a bit less.... The Military design for an SF = 2.5 instead of 1.5 for that reason.... The idea of using SF's of less than 1.5 came from the benchrest world, and they pushed down to about 1.1 (and sometimes got surprised with tumbling) for a while, but are now back using 1.5 as their "design point".... The reason is that as you drop down from 1.5 to 1.1, the bullet begins to wobble slightly, and the BC drops.... The best BC is between 1.5 to 2.5, and then as you go higher than 2.5 any imperfections in the bullet can cause wobble again.... Eventually you can get into the realm of Dynamic Instability where the bullet starts to corkscrew or spiral, but that is usually over SF = 4.0 or higher.... You have to remember that for most of our bullets, the Stability Factor follows a chart like this....
(https://oi378.photobucket.com/albums/oo221/rsterne/Ballistics/6mm%20Bowman%20Twist_zpsttaugdmk.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/6mm%20Bowman%20Twist_zpsttaugdmk.png.html)
The least stability occurs at Mach 1, and it increases as the velocity drops.... The concept that the stability increases with velocity only applies when Supersonic (above Mach 1.2).... I chose SF = 1.3 at 1000 fps because for most bullets they would still be (barely) stable at Mach 1, and in the range we look for, in the 900s, they should be right around the optimum SF = 1.5....
Bob
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Great explanation . Now I’m sure you had mentioned the minimum twist required before .
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It seems that the 51 grain .224 spitzer from NOE is missing, it would be interesting to see that added: http://noebulletmolds.com/NV/images/N.O.E._Bullet_Moulds_225-51-SP_GC_BA5_Sketch.Jpg (http://noebulletmolds.com/NV/images/N.O.E._Bullet_Moulds_225-51-SP_GC_BA5_Sketch.Jpg)
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I don't have that mould, but I will add it next time I update the database, thank you....
Bob
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Thanks Bob, it has relatively high BC on paper so it would be interesting to see how it compares.
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I think that BC is questionable, because the FF required is much too high, IMO.... but I added it to the database.... It needs a 12" twist.... I also added the 250-41 BBT, which was missing.... Database #6 is now available at the bottom of the OP....
Bob
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Bob ,
Again , thanks so much for working on this . I have spent the last few hours going through my shootings logs and updating info on chaigun for the molds I have . Great tool .
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Bob, how about updating the database to include the new slug offerings from H&N and FX? There are also some interesting molds from NOE like 218-25-RF-D6 and 225-35-RF-BY5 It would be good to see how these slugs from the tin compare to cast bullets.
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I can do that using the measurements for the NOE moulds…. I don't have any of the FX or H&N slugs....
Bob
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I can do that using the measurements for the NOE moulds…. I don't have any of the FX or H&N slugs....
Bob
I'll be getting H&N sample pack in a couple of weeks so I'll do the measurements for you if no-one else does.
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Thanks.... I need the caliber, overall length, meplat diameter, nose length (overall length minus the parallel part at the back), base diameter if smaller than the caliber, and the weight to 0.1 gr....
Bob
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I have updated the database at the bottom of the OP, to Version #7.... I have added the NOE 218-25 and 249-39 slugs, plus Bob Joatman's 358-152 HP, and the NOE HPBT version of the Lyman 257420, the 258-75 and the shortened version, the 258-63 with the BT removed.... Happy New Year to all.... 8)
Bob
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Thanks.... I need the caliber, overall length, meplat diameter, nose length (overall length minus the parallel part at the back), base diameter if smaller than the caliber, and the weight to 0.1 gr....
Bob
H&N Slugs in .217 size. Hollow point bullet with concave base. Nose profile seems to be the same for all bullets while length varies. Measures in millimeters and grams.
21 grain dia 5.51mm, length 6.70mm, meplat 3.10mm, nose 4.30mm, 1.370 grams, BC 0.91 (claimed)
23 grain d 5.51, l 7.31, m 3.10, n 4.30, 1.515 g, BC 0.94
25 grain d 5.51, l 7.65, m 3.10, n 4.30, 1.637 g, BC 0.100
27 grain d 5.51, l 8.15, m 3.10, n 4.30, 1.746 g, BC 0.95 - says so in the package
30 grain d 5.51, l 8.85, m 3.10, n 4.30, 1.954 g BC 0.106
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I have included the data for the H&N Slugs supplied by rkr above.... I note that the BC's claimed by H&N are well above the Sectional Density.... That implies that these slugs have less form drag (a better form factor) than the G1 drag model, which is below....
(https://i378.photobucket.com/albums/oo221/rsterne/Ballistics/g1_zps09zm9zd2.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/g1_zps09zm9zd2.jpg.html)
I find that a bit hard to believe, and you will note that the BC values calculated by the Kolbe Drag Calculator, and my estimates, are significantly lower.... I think we would be wise to wait until there is independent confirmation.... The new data is in Version #8, located at the bottom of the OP....
Bob
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Those H&N slugs are actually well designed IMO. The 30 grainer will fit in a 9mm magazine which means it can be used on roughly all .22 PCPs. It will also stabilize in a 19" twist while 18" is the common slowest twist rate we encounter. Even if those BCs Bob calculated do not correspond the advertised ones they allow a comparison against all the other bullets in the DB as they are all calculated the same way. It would be interesting to see how the FX slug compares if anyone has measures for them.
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Here's rather interesting reference of .22lr BCs for you Bob in case you haven't seen it:
http://www.exteriorballistics.com/ebexplained/22rimfire.cfm/ (http://www.exteriorballistics.com/ebexplained/22rimfire.cfm/)
RWS bullet shape:
(https://s3.amazonaws.com/cdn.creedmoorsports.com/images/popup/RWS2134187-ROUND.png)
Lyman 225438, which according to data in the first link should have BC of 0.2 at airgun speeds:
(https://op1.0ps.us/365-240-ffffff/opplanet-lyman-rifle-bullet-mould-22-caliber-225438-2660438.jpg)
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I have seen the Sierra data.... I don't know what drag model they are using, whether it is the G1 or RA4, but a BC of 0.20 for an SD of only 0.12 seems very high.... Perhaps the heeled base of the .22LR bullet is acting like a boattail, which would certainly help when subsonic.... Once I get enough time with my LabRadar, I will be able to test the 41.3 gr. RWS bullet to find out what the BC actually is....
Bob
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257420 BT from Arsenal, measured with labradar at muzzle speeds 300m/s and 280 m/s. Average G1 model BC is 0.17 according to chairgun.
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Thanks for that.... did you correct for your atmospheric conditions?.... Also the weight?.... Maximum distance used for calculation?
Bob
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Thanks for that.... did you correct for your atmospheric conditions?.... Also the weight?.... Maximum distance used for calculation?
Bob
Good point, the BC actually went closer to 0.18 when I used the right weather. Weight 71 grains and distance 98 meters. This was an average from 5 data sets and I used 80M and 98M values for estimation.
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I will add that to the database on the next update.... Many thanks....
Bob
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The Database in the first post has been updated to Version 9.... There are a couple of additions, and the first of my LabRadar BC data is included (in blue)....
Bob
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I have updated the Database at the bottom of the first post to Version 10.... This adds data on .30 cal pellets and slugs, and corrects some previous errors.... When actual BC testing is done, I put the Drag Model used in the "Est. BC" column.... If I am questioning the measured BC for some reason (eg. spiralling, which can add drag) I show it in red instead of blue....
Bob
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I noticed that you didn't have any 9mm listed and was wondering about things like the Lee 356-125-2R which I've been casting at .357 (+0.0002 to +0.0004). I'm still just starting testing with these to see how well they might work. I also finally acquired a Lyman 356637 that will hopefully get here soon. This is the design that they used to make the 9mm Devastator hollow point and I may machine this mould to be able to make hollow points. I'm hoping that the 356637 doesn't have too much contact with the barrel and produce really slow speeds from friction.
I've also been powder coating and sizing, my Bulldog seems to squeeze them down to .357 so I'll probably go with that size for most of my testing. But I should probably do a little polish in my .357 die, it seems to undersize just a bit, need to look at it in more detail and run more rounds through. My .358 die seems to be right on size.
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9mm and .357 are pretty much interchangeable.... If the .357 is too tight in your 9mm barrel you can size it a bit....
Bob
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I own three air rifles, a .177 cal; .30 cal Hatsan Blitz and a .50 cal Airforce Texan. I have been looking for pellet molds so I can cast my own pellets for the 30 & 50 calibers. I found molds for sale from a African manufacture that are very expensive and another from NOE who never has them in stock. I am hoping that on this site someone will be able to direct me to other dealers. Thank you in advance
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I suggest you post this question in the Cast Pellet and Boolit Gate....
https://www.gatewaytoairguns.org/GTA/index.php?board=229.0 (https://www.gatewaytoairguns.org/GTA/index.php?board=229.0)
NOE only make pellet molds up to .357 cal / 9mm.... Quite a few moldmakers do slugs in .30 and .50 cal, but if you are looking for Diabolo pellets, there is little choice....
Bob
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I have had a request to make up a chart on my new version of Bob's Boattails, with the specifications such as weight, length, SD and Twist.... I included a couple of Flat Base offspring as well.... Obviously only a small number of these are available, but it shows what could be made, if the desire is there.... I built it in Excel, but the Forum would not let me attach that, so I had to rearrange it and print it as a .pdf file.... It is attached below.... If you have any questions, please post them in this thread....
Bob
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In general nice layout, makes it easy to see if any of my hairbrain ideas have a glimmer of hope, and also confirmation as why things I tried do work.
Thanks for the cookies Bob :)
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I have updated the Database in the first post to Version 11.... I am still using G1 for the BC's because that is the most popular, however, for most airgun slugs, using the RA4 drag model.... along with the BC(RA4) of course.... will give more accurate trajectories....
Bob
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I have updated the Database in the first post to Version 12.... Ballistic Coefficients for many .257 and .224 cal slugs have been added.... I am still using the G1 drag model because is is the most commonly used, but in every slug I test now, the RA4 drag model is a better "fit".... The BC changes less over a wider range of velocities using RA4 instead of G1.... Fortunately, there is an easy to use converter you can find here.... https://www.jbmballistics.com/cgi-bin/jbmgf-5.1.cgi (https://www.jbmballistics.com/cgi-bin/jbmgf-5.1.cgi)
Input the "BC" from my Database, selecting "G1" for the input.... Input the "Calibre" and "Bulet Weight", and the "Velocity", and then select "RA4" as the "New Drag Function".... When you hit "Calculate" you will see a table, and the left lower number is the one you want, the "New Ballistic Coefficient: 0.XXX RA4".... Use that in your Ballistics Calculator after selecting RA4 as the drag model....
Bob
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I realized that I was using the average velocity as measured at the site conditions in the "Avg. Vel." column.... The BC is corrected to ICAO standard atmospheric conditions, and the velocity should have been as well.... I have gone back over all my data and made this change to the Database.... and yes, it was a LOT of work.... The previously stated velocity for most of the data has decreased about 15-20 fps, and the column is now labelled "ICAO Vel.".... The BC stated now corresponds to that velocity, as it should, from Version 13 onwards.... This change should allow a more accurate conversion from G1 to other drag models, such as RA4....
Sorry for the ongoing small error, but you won't need to worry about it in future, as I have updated my spreadsheet to report the ICAO velocity, along with the BC....
Bob
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I have updated the Database in the first post to Version 14.... I have added data on 6mm /.243 slugs, and the heavier/longer slugs in .257 cal in a 10" Twist barrel....
Bob
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Version 15 is now in the first post for download.... I have added .25 cal NOE cast pellets, and a few other things....
Bob
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Looking at the airgun bullet database, my initial tests of projectiles from a NOE 154gr BT mold have the G1 Act. BC as a higher value than any other listed projectile. G1 BC = 0.350 at 1050fps in a 1:10 twist. Nothing else even close.
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It will be partly because G1 is totally the wrong reference drag law for that type of projectile at those speeds, giving an artificially high BC value. Use RA4 or even G7.
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It will be partly because G1 is totally the wrong reference drag law for that type of projectile at those speeds, giving an artificially high BC value. Use RA4 or even G7.
That’s a great point to emphasize. I actually use RA4 for these 2+ caliber long BT slugs. For the 3+ caliber long slugs, G7 might be even better. The three models (G1, G7, RA4) have three different scale factors, so we can’t compare the values directly. Bob gave a link to a drag model conversion calculator. Knowing that G1 was inappropriate for this slug, I still used the converter to get a G1 number for comparison, since G1 is what Bob’s chart uses. RA4 is definitely a better match in this instance, especially when shot at velocities over 1000fps. The RA4 value for this slug came out to 0.270.
When I started my magnum slug journey, I was using G1 and needed to use the multi BC function in Strelok in order to get a good trajectory match for my .257 as I got close to 1000fps. The multi BC function is a work around for a poor drag model match.
I think the G1 model is only appropriate for “bragging rights” when shooting these long slugs at near sonic velocities. For actual trajectory calculation, we should use a more appropriate drag model.
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Interesting point, Scott.... When I started my database, we weren't pushing velocities much over 900 fps, and it was pretty much the only drag model used.... As you and others have proven, the G1 is a poor match for airgun slugs when shot at over 1000 fps.... I don't have the time right now to convert all this data from G1 to RA4, but it would probably be a much better match....
Being able to have a BC (G1) of 0.350 definitely gives you (us) bragging rights, though!.... 8)
Bob