I finally had some time to do some conversion for various calibers of the Ataman M2R Carbine Air Rifle using the energy specs listed on a sellers website. I chose rounds of comparable shape for all calibers, got the weight of the pellet and ran the calculations. Here are the results:
.177 - EFactor 3
.22 - EFactor 3
.25 - Efactor 4
.30 - EFactor 4
.357 - EFactor 5
Since these numbers assume very good quality control and precision manufacturing processes, survivor variants may have lower EFactors.
For a comparison, I also looked up the FX Crown. The converted EFactors for it are:
.177 - EFactor 4
.22 - EFactor 4
.25 - Efactor 5
.30 - EFactor 5
These numbers seem to be rather consistent with modern air rifles.