40mm aluminium TV aerial pole about 35m away from my back bedroom window - use the sniper technique of opening the window and firing from well back inside the room so i can't be seen from outside. It makes a nice 'ting' when hit and being able to hit it within 3 shots is my minimum requirement for accuracy from my guns.
This may sound quite bad and, trust me, i'm not promoting it as a good option, but it is out over rooftops, and there are no windows facing my way, so nobody could actually be hit by any BB's, but i expect that many people, including the police, would not see it that way...
Sticky targets are good for pistol practice - not so much for AEG's, especially on full auto... although that is useful for proving that all BB's shatter, no matter how expensive they are.