go down to the county court house and get a copy of zoning regulations. There has to be some sort of documentation concerning this, if not, wait until the next council meeting have an ordinance put before the council for a vote to become law.
My concern would be civil more than criminal.
My guess is that the officer was threatening based on civil responsibilities more than criminal code. Go to the Sheriff dept. and talk to the boss, see what he says and if there is any local, county or state codes that you should be aware of.
You do have to show " do diligence " in building a back stop to show that no rounds leave your property. If a round left your property and caused damage or injury, you could be held responsible for being negligent.
You get out in the stick, most people do just go out and shoot in their back yards - when I'm at my place in MO - I do all the time, BUT I am aware that there are cattle grazing in the fields around me and if a round hits and kills one, I will be buying a bunch of steaks