I've had that same thing happen on my property , minus the "Shootem-up" part.
My wife and I live up on a mountain and on both sides , run streams. We've had campers and hunters set up base camps for hunting or whatever.
I was wearing a G17 on the left side and a G36 on the right when I went down to confront them . They were quite polite and understanding.
I don't own a huge parcel but it's plenty . Even got two shooting ranges, one 75 YDS.deep x 55 YDS. wide and the other 200 YDS.deep x 55 YDS. wide...I love it , 40 YDS. from my front door !
The one group of campers was so polite , we have become friends.That there stream , (McCunna Run) , part of it's mine and does seem to attract the way-farer , hunters and fishermen . If you come , just let me know you're there so's I don't have an accident.