Well, depending on where you live you can be seriously screwed over by the local authorities. Check with your local By-Law enforcement office and ask what the By-Law regulations are for discharging an air gun.
For example, anyone with in the GTA (Toronto, Scarbarough, Etobicoke, Mississauga, Brampton, etc) will want to avoid shooting an airsoft gun in their back yard, because there is an existing By-Law stating that it is unlawful to discharge any air gun (paint ball, airsoft, BB gun, etc) with in city limits in the GTA area. So if your neighbours call in and complain, the locals police will have grounds to seize your toys, and possibly charge you with disturbing the peace. If on the other hand you are lucky enought to live in the country, you should be golden.