once they hit the ground most bbs will go poof and disappear with a flash of blue scented smoke
however be warned about those nasty ones that linger around, they'll multiply into colonies of un-poofable bbs
oh and btw if you search "biodegradeable bb" in the search function, at least 5 topic comes up:
http://www.airsoftcanada.com/showthr...biodegradeable (this one being almost EXACTLY what you asked)
the following discusses biodegradeability of specific brands
http://www.airsoftcanada.com/showthr...biodegradeable
http://www.airsoftcanada.com/showthr...biodegradeable
http://www.airsoftcanada.com/showthr...biodegradeable
http://www.airsoftcanada.com/showthr...biodegradeable
[end spoonfeed]