Um... Have you approached the property owner at all?
It could be that the property owner is unaware of the teens using that part of his/her property for such purposes.
Just make it clear that you consider their firing in the direction of your house at such a close distance as unnecessarily dangerous. Ask the owner to prevent this kind of activity on his property for your peace of mind. And his.
If the property owner refuses, I think I might place my request in writing, delineating the risk and noting that he had been asked to prevent the risk and refused and that he/she would be liable for any property damage or personal harm arising from such firearm activities he/she had been asked to terminate. Send a copy to him/her and to your attorney.
Attorneys: Couldn't this kind of behavior be considered "reckless endangerment"?
I'd also be tempted to "return fire"....that is, do some target practice into the opposite side of this berm that the teens are firing into, preferrably simultaneously to their discharge activity. Rash, but given the unwillingness of the authorities to terminate a high-risk situation, it might even out the risks and make it clear to them (but I doubt it). "Mutual Reckless Endangerment - MRE". 'Course, you probably don't have this option available, huh?
Also.... Bring this issue to the attention of your state representative and your county commissioner. They have the responsibility to legislate safe and reasonable gun use...despite what the NRA may believe. You are being placed in a situation of unnecessarily high risk because of the inaction of your elected representatives.
(see here:
http://www.motherjones.com/news/spec...a.sidebar.html)
godfry