I seriously don't think I'd want to try killing something at 600 yds with an air rifle.
The length of the bullet probably has to be at least twice the diameter for in-flight stability at the desired range plus being lead = significant weight. Even in a standard handgun .45ACP, 230 grain is the usual weight so a rifle round being heavier is not surprising.
I seriously don't think I'd want to try killing something at 600 yds with an air rifle.