This would not be a self defense scenario no matter how you looked at it. Someone stealing your property (scrap metal) does not warrant lethal force. Like Nomercy said the DA would have no problem convicting you on murder charges if you were to shoot someone on your property that was at a distance over say 7yds. That is the legal distance where shooting someone in self defense is justifiable. Anything over that and you are looking like you were intentionally out for blood.
I understand your frustrations as to me there are few things in this world that are lower than a thief. I have had a vehicle and other personal property stolen and I would have love to have been able to shoot the SOBs. Unfortunately it is against the law to do it.
Your best is to install cameras and motion sensing lights. Even posting one of those signs that say smile you're on camera works. The contractor on one of my projects was having problems with theft and he posted a few of those signs up and the theft stopped. I asked him if he really had the cameras and he just said no but they don't know that I don't have them.