I really hate to hear about cases like this. Around here our police department is top notch. I can call and complain that kids are knocking down mail boxes and they will come running out to investigate. Stolen property is taken very seriously. Then again, I live in a relatively crime free area and all the cops have to do around here is take care of minor theft issues. Nothing major happens here. Unfortunately, the State Police in most states are a completely different matter. They have their hands full with all kinds of nasty stuff that you would not even want to know about. Sad but true. It stinks that you are not getting the justice you deserve.
However, shooting at people and setting booby traps are very bad ideas. Is that metal worth your life and liberty? Probably not. The game camera idea is good as is just being more careful to secure your belongings. Criminals are always on the lookout for "low hanging fruit". Evidently your setup is such that they see you as an easy victim. If metal is laying out in a field in plain sight then you are going to be a target. I'm not trying to lay the blame on you. After all, you SHOULD be able to just put a gas can or metal on your property and not expect it to disappear. But the reality is that we have to protect ourselves and our stuff if we don't want to be victims.
Take a look at your setup and how you can stop being seen as an easy victim. Criminals want to do the least amount of work possible for the greatest rewards. They aren't going to go for high risk, low reward targets.