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rsterne · Moderator

I have been doing quite a bit of research on how the Ballistics Coefficient can be calculated for various bullet shapes.... and I find out that in simple terms, the BC may be estimated by dividing the Sectional Density by a "form factor".... It turns out that for spheres, the form factor is 1.55 (although no velocity was stated).... Since the SD is an easy calculation, I thought I would work through the math for various calibers to see the effect that caliber has on BC.... Logic dictates that whatever applies to spheres should also apply to pellets, as long as the shape is consistent.... so the results should give us some indication of what to expect of the BC as we increase the caliber.... Here is a graph showing how the BC varies with caliber....

The weight of a round ball that fits the bore varies with the cube of the diameter and is shown on the graph.... The Sectional Density is found by dividing the bullet weight (in lbs.) by the bore (in inches) squared.... For a sphere, the SD varies in direct proportion to the caliber.... Since the Ballistics Coefficient is the SD divided by the form factor (1.55 for a sphere) then the BC of a round ball also varies in direct proportion to the caliber.... In the graph above, the calibers on the horizontal axis are not evenly spaced, but if they were, the graph for the BC would be a straight line as indicated by the black trend line....

If we look at what effect this might have on round nosed pellets like the JSB Exact series, we can reach some interesting conclusions from a theoretical point of view.... Most of the Exact series have very similar shapes, and so the Ballistics Coefficient should be closely tied to the Sectional Density.... I have calculated the SDs for all of the Exact series below....

.177 Exact RS 7.3 gr. : 0.033
.177 Express 7.8 gr. : 0.036
.177 Exact 8.4 gr. : 0.038
.177 Exact Heavy 10.3 gr. : 0.047

.22 cal Exact RS 13.4 gr. : 0.041
.22 cal Express 14.3 gr. : 0.043
.22 cal Exact 15.9 gr. : 0.048
.22 cal Exact Heavy 18.1 gr. : 0.055
.22 cal Exact Monster 25.4 gr. : 0.077

.25 cal Exact King 25.4 gr. : 0.058

The first thing that jumped out at me is that the SD for the .22 cal Exact 15.9 gr. is almost identical to that for the .177 Exact Heavy 10.3 gr.... According to the pellet database in Chairgun, they do, in fact, have identical BCs of 0.031.... That is consistent with my measurements as well (at similar velocities).... We don`t yet (AFAIK) have a published BC for the 25.4 gr. JSB Exact King in .25 cal.... However, it`s SD is just a bit better than that of the .22 cal Exact Heavy 18.1 gr.... and I have measured that pellet`s BC as high as 0.043 at 850 fps.... It is therefore reasonable to assume that the BC for the .25 cal JSB Exact King will be at least that good.... Mr. Green

Bob