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

Bob's Boattails Drag Calculations

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**rsterne**:

I have asked British Ballistician Miles Morris to run my current Bob's Boattail designs through his program to calculate the Drag Coefficients.... Here are the results for the six different length designs.... I have included the common G1, G7 and RA4 drag models for comparison.... Note that the commonly used G1 drag model is a very poor choice for all these slug designs, as it predicts about 25% more drag at 1000 fps than occurs with most airgun slugs....

Note that below Mach 0.8 (900 fps) the two shortest BBT designs, the 1.5 calibre and 2.0 calibre long versions, have virtually the same drag.... They are a good fit to the RA4 drag model (originally intended for the .22LR bullet).... Here is another chart, showing the velocities over Mach 0.8 in expanded scale for more clarity....

It is interesting to note that the longer designs, 3.0 calibre, 3.5 calibre and 4.0 calibre long BBT, are a near perfect match for the G7 drag profile, which makes sense, as it is intended for boattail bullets!.... They are also very close in drag to each other, with the 4.0 cal. long BBT having only a bit less drag that the 3.5 cal. long or 3.0 cal. long designs.... The middle length, 2.5 calibre long BBT, is a decent fit to the G7 drag profile, but Ballistics Coefficients calculated using that would have quite low values (not an issue if you use G7 for the trajectory calculations).... Scott Hull, who has been shooting a 7mm version of that slug (weighing 106 gr.) has found that using the RA4 drag model works very well below Mach 0.95 (1070 fps), at distances out to 500 yards, so that would be a good choice for the 2.5 calibre long BBT.... For those of you not familiar with my BBT designs, here are 3D printed models of them, all to the same calibre (in this case 51mm) ::) ....

These Drag Coefficients are calculated, not measured, but Miles feels they should be within 5% of the actual drag.... I am in his debt for taking the considerable time to run all these through his ballistics prediction program....

Bob

**jackssmirkingrevenge**:

--- Quote ---all to the same calibre (in this case 51mm)

--- End quote ---

I look forward to your next big bore project ;D

**rsterne**:

Jack, it might be possible to shoot those plastic models, but with the many hours to print, ammo supply would be a problem, and the BC would be terrible because of the light weights.... The models were .257 calibre, scaled up almost 8 times for printing.... Each 0.2mm thick layer in the print represents 0.001" in length on the .257's.... The 4.0 cal. long model is just over 8" tall.... They sure do look nice sitting on my bookshelf, though.... ;D

Bob

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