Biologically degradable bbs are more respectful of the environment especially if your airsoft field is an open space and not a CQB site. They are recommended and sometimes required in airsoft associations. Airsoft bbs are available in different weights to match each type of replica. For example in a sniper rifle, it is better to use "heavier" bbs (from 0.28g to 0.45g) that will allow more resistance to the wind and provide a more stable trajectory for your shots. For a normal use with a standard replica, 0.20, 0.23 or 0.25g bbs are recommended.