Do what you have to do Brian. Since you'd be supporting Bastard products, there would be few here who object to doing that.
For the people who blatantly disregard your request for use of .20's, boot them. Refuse them entry at future events. If it's to ensure the safety of other players, by all mean force people to by ammo. If it's to prevent contaminants of a land not owned by you, by all means, sell bio bastards.