B&B Havin' Fun
It appears that their "backstop" is a stand of trees. While there are a few unknowns such as the depth of the stand of trees...distance beyond the trees to the nearest house, highway, place of business,etc...and it's not highly likely that any of the rounds could get through the trees...anything can happen. Murphy lurks everywhere, looking for a situation in which to apply his universal, inescapable law.
In short...No way would I depend on a stand of trees...even a thick stand...that was less than 500 yards in depth to stop and contain that many rifle rounds. Even in the thick brush that makes up the majority of Southeastern Whitetail hunting...a bullet has been known to thread its way through to frightening distances. The chances of one making it through is small. Multiply the odds by the number of rounds fired, and it gets one's attention.
My vote goes along with the Beavis and Butthead observations. This bunch coulda done better.