Airsoft guns fire a 6mm spherical BB. The BB can be made from a variety or plastic or biodegradable materials depending on the manufacturer.
BB’s are most commonly white, but are also available in other colours.
There are many different grades of quality and weight, with the better quality BB’s being recommended to avoid gun issues. The most commonly used weight would be 0.20g and 0.25g. As a rule, the higher the weight, the more accurate the BB (outside) but the less distance it will travel.