My knowledge as a layman would be most laws governing this are on the municipal level and hunting and target shooting are two different venues. In short, you might have some legal ground to curtail some of his use of of HIS land and you might start a turf war that no one wins. Research your legal status so you know what ground you have to stand on and speak with him and explain your position as a neighbor and would be friend. The old axiom of catching more flies with honey than vinegar comes to mind in a situation like this. Remember that this is all new to him and over the years I have witnessed people come and go and things change as the new novelty wears off. Your legal standing on this will lend to your bargaining power but keep in mind the above mentioned axiom even if the law is on your side. You might be able to stop or curtail the shooting but an off-road atv and dirtbike track might not be so attractive either....As far as the use of roadways by an atv, to my knowledge that is illegal-stop that and I'm sure he has a car...Carrying an unloaded long gun on an atv is legal as far as I know but enconn PD or CSPD would have the final say. Good luck, too bad you didn't purchase the property but like I said, things change maybe it will come back up for sale. Where are you located if I might ask???