I don't understand why you would not take any action??? Your tenant, your responsibility. Your neighbors complained to you. I am not a lawyer, but I would bet that if someone gets shot you WILL get sued. Sounds like classic civil liability at the least. You should consult an attorney and take action to protect yourself. Maybe send a cease and desist letter followed by an amendment to the tenant agreement. If they won't abide by it then send them packing. Losing a few months rent is better than the alternative. Talk to a lawyer before the worst happens.