Mu club built a backstop made mostly of old Railroad ties. Then one spring, the farmer who leased the property decided to burn the weeds, and his fire spread to the ties and burned them down,too. We had many pounds of melted lead, which the guys split up who had casting equipment to make into balls.
If your backstop is a dirt mound, over the years the dirt will slide down, and wash out the lead bullets and balls. Often you will find a large chunk of lead where all the balls landed, and it only take washing it off thoroughly and then drying it thoroughly for several days in the sun to separate the lead from the dirt. Sand traps work, but a maintenance nightmare. If it gets wet, it now only gets heavier, but it swells and can break its contain after a time or 10. If you put in wet sand, it settles quite a bit when it dries. So, you will need to keep an eye on it. The larger the ball into the box, the more sand comes out. I have tried boards, plywood, paper, and combinations of them to extend the useful life of a box of sand, and concluded you just had to expect to shovel more sand every time you wanted to use it. A sand pile is good, if it is large enough. That's the rub. The rain will disburse it every time it rains, and that 12 foot tall pile will quickly be half that size. Obviously, it is less a problem if you build a shelter for the pile, and that is what some ranges do. Then they use bulldozers to push the sand and dirt back up into the pile every so often to keep the height up where its needed. Good luck.