The problem with asking for advice about a gun range on a public forum, is that anyone and everybody with an axe to grind feels obligated to give their (expert) opinion.
Stick around, and pretty soon someone will start talking about magic bullets that bounce off of rubber tires and how the steel belts in tires will cause ricochets, or how lead will aggregate in sand and cause ricochets.
The NRA guidelines are more for professional/public gun ranges, not private ones.
The only gun range I have ever seen with a twenty foot high backstop, was a police range, which was built with tax payer dollars. It was built to NRA Guidelines. The private range I have shot at doesn't have a twenty foot backstop, I would estimate it as 12-15 foot high.
I am fortunate to have plenty of land to shoot on, and don't have a formal shooting range or backstop. I can use the hill or pond dam as such when needed.
The best advice is to use common sense. How close are your neighbors? What type of land surrounds yours, and what are the natural features (if any)that may be incorporated into a backstop?
Look around and talk to other people you know who have a private gun range, or have shot at one. Go online to YouTube, and look at what some of the posters there use.
Dirt piled up in a mound is probably the best backstop, but sand works as well, and using a combination of dirt against a wooden wall would cut down on the amount of dirt needed to be piled up.
Tires stacked staggered two deep, and filled with dirt or sand also will work, but require a lot of work.
Use your imagination, but avoid hard surfaces such as concrete or rock unless
covered by a layer of dirt.
Also avoid steel targets unless you have plenty of room to build enclosing wings and baffles. Steel targets
will ricochet!
