Author Topic: Cast Bullet Database - A Work In Progress  (Read 1202 times)

rsterne

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Cast Bullet Database - A Work In Progress
« on: January 23, 2019, 08:02:35 AM »
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 mostly blank columns for the actual BC @ specified velocity.... There are only a few measurements I have input so far, but I hope to gradually 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 5 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
« Last Edit: February 23, 2019, 06:22:08 PM by rsterne »


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rsterne

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Re: Cast Bullet Database - A Work In Progress
« Reply #1 on: January 23, 2019, 08:08:22 AM »
Here is the G1 Drag Model, showing how the Drag Coefficient increases with velocity....



Here is the exact same chart, with the Cd plotted against the Mach number instead of fps....



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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rsterne

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Re: Cast Bullet Database - Roundball BCs
« Reply #2 on: January 23, 2019, 03:11:12 PM »
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....



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....



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
« Last Edit: January 27, 2019, 09:57:37 AM by rsterne »
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rsterne

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Re: Cast Bullet Database - A Work In Progress
« Reply #3 on: January 25, 2019, 09:56:59 AM »
Database in OP updated, a couple of small errors corrected, .25 cal, .30 cal, and .36 cal buckshot added....

Bob
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Re: Cast Bullet Database - A Work In Progress
« Reply #4 on: January 25, 2019, 05:36:53 PM »
Thank you, Bob. Just started casting my own bullets for the first time this week. Haven't had a chance to shoot them yet (nor the BBT's that Ricky sent me) but this casting thing can get addicting.
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Re: Cast Bullet Database - A Work In Progress
« Reply #5 on: January 25, 2019, 05:46:17 PM »
Yes it can. I was at the reloading store today looking at another mold while I was picking up 2lbs of BP.
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rsterne

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Re: Cast Bullet Database - A Work In Progress
« Reply #6 on: January 27, 2019, 10:07:07 AM »
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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Re: Cast Bullet Database - A Work In Progress
« Reply #7 on: January 27, 2019, 05:36:51 PM »
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

I’ve noticed that there are some doubters about your numbers, and scientific method of predicting performance based on bullet size and shape, barrel twist rates, psi etc  vs “real world” experience. Sure seems that the variables can be accounted for, and calculations result in information to help us all chooose a better bullet from the get go.

This is really good stuff!
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rsterne

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Re: Cast Bullet Database - A Work In Progress
« Reply #8 on: January 27, 2019, 07:04:49 PM »
There is nothing to substitute for hands on experience and measurements.... I've drawn from 3 different sources here, the manufacturer's BC claim (where they make one), the Kolbe Drag Calculator, and then I've tried to come up with my own system.... The values I am getting are in between the Mach 0.5 and Mach 1.0 values from Kolbe, which makes sense.... Only testing, and then refining the numbers, will help us improve our understanding of the BCs....

I hope eventually to develop a couple of new "drag models", one for pellets, and one for the common bullet shapes we use in airgunning…. a nose with a tangent Ogive and then a Meplat of 50-70% of the caliber (and sometimes smaller in bullets intended for long range)…. I think the LabRadar will be the best tool for the job, which is why I am saving the small fee I get from HAM for my monthly articles towards the purchase of one....  ;)

Bob
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rsterne

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Re: Cast Bullet Database - A Work In Progress
« Reply #9 on: February 01, 2019, 09:20:41 AM »
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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rsterne

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Re: Cast Bullet Database - A Work In Progress
« Reply #10 on: February 23, 2019, 06:21:41 PM »
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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Re: Cast Bullet Database - A Work In Progress
« Reply #11 on: February 23, 2019, 07:56:47 PM »
Thank you Bob! I greatly appreciate all the time and effort you put into this project!

On a different subject, any updates on Noe .257 Hollowpoint bullets in BBT?
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rsterne

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Re: Cast Bullet Database - A Work In Progress
« Reply #12 on: February 23, 2019, 09:54:01 PM »
Nope, no word for a while, unfortunately.... Al has been strangely quiet recently....  ???

Bob
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