If yo read the comments here and in the past that Double Naught Spy has written, you will see that he is not being a smarty pants and trying to rain on anyone's parade. He has valid comments and sharing them with us should make us think.
All firing range berms need maintenance. He has pointed out where the current berm may fail. If the OP decides to inspect the berm and confirm the backstop could be built better, good for him. It sounds as if he will be operating some sort of business and this is one more thing he will have to pay attention to.
I like the OP's backstop. I may build one like it eventually, but I will keep in mind what DAS has written. It will be interesting to see how many rounds the backstop catches before it has to be redone. (Rubber replaced/different material used/etc...)
Inside Every Bright Idea Is The 50% Probability Of A Disaster Waiting To Happen.