I would go with topsoil, once it is packed the bullet wont go any deeper than an inch or 2. I use to shoot a 7mm,270,and a 243 at a dirt mound and we found almost every bullet just inches into the dirt. I would make the mound pretty big or construct a 8ft wide by 5 ft high by 3ft deep or back and fill it with dirt. This will do the trick.