The tube does not need to be 500 yards. It needs to be long enough to allow ricochets to deflect and throw the shrapnel down range. The shooter can have as little as a 24 inch wide pipe fixed at the bench (maybe 50 feet long) with only the muzzle in the tube (you dont want the action in there, it gets real loud real fast).
There was something similar to what you describe at Cannon AFB in the late 70s and early 80s. There was a gong at the other end; shots were instantly confirmed. I saw another one (on private land) that was the same way, but much longer than Cannons, the lane was walled up with 'jersy barriers' 3 high and bermed up with earth on the outside. Angled metal plated every so often kept zingers from flying out the top.
I am thinking (off the cuff, so dont flame too hard if this doesnot work out...) that if you had the first 50-75 meters or so of the lane built as described above, and then the last 20 meters or so at the target built, chain link could take up the rest, just to keep critters/people/whatever from wandering into the lane. The initial tube will keep the boolit frags going in the right direction. Unless there is a balloon or egg factory right next to the lane, everything should be fine.
Anyone been to the range at Cannon lately?
Edit: "The Tube Range" gay bar name