How much land do you have avalible? Rifle bullets WILL bounce off a earthen berm, and travel quite a ways. If there's no trees or terrian avalible, think long and hard about where a bullet may land. From running military night fire ranges, I'd guestimate that about 5%+ of M4 rounds fired at a 12' berm wind up flying outside the confines of the range and into the impact area ( which is well over 1k acres). While its a low amount or rounds escaping, and even lower odds of hitting people or damaging property, it is a risk that needs to be considered.
The range I'm working on (which I can't take credit for, as the designer is the facilities manager at CAG) is concave wall of tires, 70' from end to end, 18' high, and about 15' deep. The tires are stacked 3 abreast, and filled with sand clay as they're stacked. the inside of the wall will be backfilled, and there'll be an 8' plate steel "roof" to catch any flyers. The tires and clay are free, the roofing will cost a couple grand. Well worth it in my opinion.