Author Topic: Cast Bullet Database - A Work In Progress (Read 18925 times))

rsterne · « **on:** January 21, 2019, 07:31:53 PM »

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

rsterne · « **Reply #1 on:** January 21, 2019, 08:10:53 PM »

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

Hobbyman2007 · « **Reply #2 on:** January 21, 2019, 08:11:40 PM »

Nice work Bob . One question ........ do you ever sleep ?

darkcharisma · « **Reply #3 on:** January 23, 2019, 01:38:52 PM »

Bob, here's more work for you. Give us the blue prints or the dimension drawings. 😁

Thank you in advance!

rsterne · « **Reply #4 on:** January 23, 2019, 02:14:28 PM »

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

MI Yeti · « **Reply #5 on:** January 23, 2019, 02:32:33 PM »

Very cool. Nice to see the lee 340-f that I shoot in my texan in there. What lead alloy was used for it?

rsterne · « **Reply #6 on:** January 23, 2019, 04:07:18 PM »

Probably pure lead or 1% tin.... I now use 40:1 but that was a while ago....

Bob

rsterne · « **Reply #7 on:** January 23, 2019, 07:12:26 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

PeterL · « **Reply #8 on:** January 23, 2019, 10:27:46 PM »

Thanks for the great data Bob. So, would the boattails that Nick sells be considered spherical(GS drag function) or the universal G1?

Peter

rsterne · « **Reply #9 on:** January 24, 2019, 12:17:32 AM »

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

PeterL · « **Reply #10 on:** January 24, 2019, 09:24:22 AM »

Quote from: rsterne on January 24, 2019, 12:17:32 AM

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

rsterne · « **Reply #11 on:** January 25, 2019, 01:58:20 PM »

Database in OP updated, a couple of small errors corrected, .25 cal, .30 cal, and .36 cal buckshot added....

Bob

rsterne · « **Reply #12 on:** January 27, 2019, 02:06:36 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

KnifeMaker · « **Reply #13 on:** January 29, 2019, 05:54:30 AM »

Wow! My head hurts! Whew!!!

Thanks a Million Mr. Bob!

Mike

rsterne · « **Reply #14 on:** February 01, 2019, 01:22:28 PM »

Database in OP updated again to Ver.4.... Weights added for many of the NOE Pellets, and a couple of other minor changes....

Bob

KnifeMaker · « **Reply #15 on:** February 07, 2019, 09:50:43 AM »

I spent another hour looking at the list. And I've been up all night in the shop! LOL!!!

Once again, thank you Mr. Bob for all your hard work and efforts!!!

Mike

rsterne · « **Reply #16 on:** February 23, 2019, 10:21:10 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

Hobbyman2007 · « **Reply #17 on:** April 26, 2019, 07:44:20 AM »

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?

rsterne · « **Reply #18 on:** April 26, 2019, 11:48:26 AM »

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....

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

Hobbyman2007 · « **Reply #19 on:** April 26, 2019, 07:08:00 PM »

Great explanation . Now I’m sure you had mentioned the minimum twist required before .

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Cast Bullet Database - A Work In Progress

Author Topic: Cast Bullet Database - A Work In Progress (Read 18925 times))