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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Cast Bullet Database - A Work In Progress

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rsterne:
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

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

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

Thank you in advance!

rsterne:
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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