Sturtle.
I have a berm at my house. Please, for the love of GOD, don't use tires.. I tried the same thing you are describing and had a .45 come flying back at my head. Missed me, but I swear I peed just a little.. You will not get riccochets from dirt unless it has rocks in it or it gets packed down too much. Build your berm as high as possible. It cannot be too high or too wide. Good luck!