You can get a rough idea of the Subsonic BC by calculating the Sectional Density.... A "typical" bullet will have a BC of about the same value.... The SD of a 310 gr. bullet that is 0.44" in diameter is 0.229.... I would think that the 0.218 BC quoted is probably close enough, particularly at ranges of only 60 yards.... The actual value will depend on the shape of the bullet....Bob
If the bullet is 0.429" diameter, the SD of a 310 gr. bullet would be 0.241....Bob