Even if you could complete the suggested modifications, and have your local Registar of Firearms acknowledge that is it no longer a firearm per se. Even if you could get permission to take out of your house and not to an authorized range, and too an airsoft event. I would object to its presence on the field. I would in no way participate in an event where a firearm that takes real projectiles that has been modified to airsoft bbs. I think that the possibility of a possible safety related event or a legal situation would present itself way to quickly.