I can see this as entirely believable. Yes I saw the argument about how nobody would do such a thing, or that the fact they won't mention what group proves it being BS, but think about it. It is entirely understandable why they wouldn't say(think LAWSUIT or even threats from said organization of a gag order or be sued). At a local shotgun range for trap, clays, and skeet, a similar incident occured. Although the person didn't belong to an antigun group, and the circumstances were a bit different but this is how it went.
Houses are about 200-300 yards away in the direction you shoot. Ammo is limited to VERY light loads, they sell the only loads allowed. At about 50 yards or so maximum the shot its the ground. There is even the boundry of how far the shot can go marked out. Interestingly enough you never see any wadding, or even bits of the clays past that point, quite the opposite it's all at least 15 yards inside that boundry. Anyway some buffoon, who had been trying to say having a shotgun range was dangerous, bought a shotgun, slipped a slug in, and shot at one of the houses. Then went and reported it to the media, stating that the people who lived there were startled when a round went through etc. The problem is the house in question luckily the family was on vacation, and the guy got found out. His whole theory was shotguns were the easiest target due to their extra lethality. Another victim of hollywood genius.
Rev. Michael