Title 34 may have part of the solution during big game hunting seasons:
Sec. 2507. Restrictions on shooting.
(a) General rule.--It is unlawful for any person during the open season for the taking of any big game other than turkey to:
(1) Shoot at any mark or target other than legal game or wildlife with a firearm of any kind or a bow and arrow.
(2) Discharge at any time any firearm or release an arrow at random in the general direction of any game or wildlife not plainly visible for the purpose of routing or frightening them.
(3) Discharge at any time any firearm or release an arrow at random or in any other manner contrary to this section.
(b) Exceptions.--This section shall not be construed to apply in any manner to:
(1) The discharge of any firearm for the sole purpose of signaling for aid or assistance while in distress.
(2) The use of rifle, pistol or archery ranges owned, leased or maintained by a State or Federal military or police organization or by any regularly organized rifle, pistol, shotgun or archery range, shooting association or club while shooting at a regularly established and properly safeguarded range or to any public shooting exhibition properly safeguarded and conducted under the direction of any organization for the promotion of marksmanship.
(3) The discharge of a muzzle-loading firearm at a proper target for the purpose of safe transportation of the muzzle-loaded firearm.
(4) Shooting at a properly constructed target or mark or a dead tree protected by a natural or artificial barrier so that the ball, bullet or arrow cannot travel more than 15 yards beyond the target aimed at, after making due allowance for deflection in any direction not to exceed an angle of 45 degrees. Target shooting shall only be lawful when it is done:
(i) Upon property owned by the shooter or by a guest of the property owner.
(ii) Within 200 yards of the camp or other headquarters where the person shooting is quartered or is an invited guest or visitor.
(c) Penalty.--A violation of this section is a summary offense of the fourth degree.
From your description, the firing of the weapon other times of the year is a larger and continuous problem that needs to be dealt with. Talking to the people is the first step.
Property rights are what they are beyond that. They have no right to infringe your enjoyment or use of your property, nor to inflict any damage to your property by shooting into it.
If talking does not resolve the situation, seek legal help.
I would suggest strongly that the firearm safety issues you pose be front and center of any discussion with the neighbor. From the sounds of it, they have no way of knowing when someone is in the down range area they choose to use. The lack of any back stop is just asking for problems.
I would be talking to them following up with a certified letter, also recorded it eh county records, stating the demand to cease and desist. State the legal and safety issues. If something ever does happen, you have far more legal recourse to go after them.
We all love guns and using them. That does impose certain responsibilities. Failure to adhere to them has a cost.