Some explanation is in order. HAM does not list a BC for .20 caliber, at least they didn't last time I looked. All BC values except for the JSB .20 were copied from the HAM tables, which are based on the GA drag model. I measured the JSB .20 BC myself using a Daystate having an average muzzle velocity of 835 fps, and came up with a G1 BC of 0.0385. This was then converted to a GA BC of 0.036 to allow comparison to the rest of the values in the table. Conversion is possible because the two drag models have essentially the same shape in the region of airgun velocities, one is just a bit offset from the other.
At the time, Motorhead was reporting BC values as high as 0.042 for the JSB 13.73. I assume that was a G1 value. I went with my G1 value of 0.0385 for the table to be conservative, and also because I could back that up with my own data.
In any case, the argument for .20 caliber revolves around BC, and if new testing supports a higher BC than what I measured, it only strengthens the case.
As an aside, it is not as easy to measure BC as some might think. A head wind or tail wind will affect the results, and so will pellet stability. If the rifle being used does not fully stabilize the pellet over the distance in which velocity is being measured, apparent BC will suffer.