Today I tried the friendly neighbor approach. I saw the "gunsmith" out in the yard and went over to talk to him - asked him if he had any luck deerhunting. Also talked about trimming the grass along the fencerow. He asked me not to trim it so there would be some cover for rabbits - I agreed not to.
Then the $64,000 question: "What can we do to make the downrange situation better when you're shooting? "
His reply - "it's just fine the way it is". He said he designed the 3' x 3' backstop himself with a hay bale, a layer of 2x6 lumber, 6" of sand, and some 1/2" steel plate. He states that he and his friends "never miss the target", so there "wouldn't be any downrange problems". I said "if that's the case, how about I help you move it over to the other property line". "No, I don't want to move it, cause that's where I have it set up; besides there's a house downrange on that lot line". (That house is over 1500' away, and 250' off line, as opposed to my house that is 450' away and ~150 feet offline.) His unwillingness to even consider any other solution was becoming evident. I then suggested that since he "never misses", and there are no downrange problems, how about he turn his range around and shoot the other way. He seemed shocked - "I'd never do that! - that's towards my house!". I expressed that there wouldn't be a problem since he "never misses". We discussed having the township's gun safety officer stop by to get their opinion about his range. "You can do anything you want, but I'm still shooting there". It's clear where he stands.
I left the NRA rifle range development material with him - it could be useful - he could use the fifteen pages to light his fireplace. Overall, I was fairly discouraged with our interaction.
Previously someone mentioned that a 12" pine tree wouldn't be enough to stop a high powered rifle slug. How about 10" of concrete?