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

Bob

rsterne:
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:
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:
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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