It sound like to me that you need a better back stop.
One of the basic tenets of safe shooting is know your back stop. Key word stop. If you are only seeing hits on trees and skips on the ground you haven't found the back stop yet. You must know where these ball are going and where they are stopping or you are not shooting safely.
Visit some of the several post on this subject by Seacoast Artillery on this subject for how far these thing can travel. They shoot on the wide open prairie and can see for miles and have found projectile a long ways from where they were shooting.
Yeah, Yeah, Yeah, nag, nag, nag....but its about safety and any cannon accident gives us all a black eye, so the nag will continue.