NRA publishes a book that provides information on how to build a safe range/backstop.
The manual states that the slope of a dirt berm backstop should be as steep as possible, but not less than 45 degrees (1:1 slope). The manual also advises removing all rocks and other materials that could cause a ricochet.
It states that while natural hills can often be used as the basis for a dirt berm backstop, shaping the impact side is generally required to achieve the proper minimum slope.
The link is to an article that quotes the NRA Range Manual.
http://nssf.org/ranges/RangeResource...y%20Management
Your backstop is high enough, but the angle isn't steep enough.