Chances of a ricochet while practicing?

I've taken a few training courses and have found a great outdoor range to practice what I'm learning. The backstop at the handgun range is just a dirt mound - more like a natural hill. If I'm doing close range practice, what is the chance of a round striking a rock and ricocheting back? This is a heavily used range that has been in operation for decades so I assume the chance is virtually zero.I ask because a long time ago I had a friend die from a 22 round ricocheting off a rock and striking him in the temple.Edit: I practice with a 9mm and a .380 via /r/CCW http://ift.tt/2tnotqz