I've helped build several places to shoot and a excellent bullet stopper is used semi tires.
Trucker tire shops are usually glad for you to take them off their hands, I'm talking about wore out ones.
You can lay them on their side and fill with sand or dirt, fill the whole tire up not just the middle and stack them 2 or 3 high and you will have a backstop to last for a long time, and when you do shoot a hole big enough to matter you can rotate the filled tires a 1/8 or 1/4 turn and shoot a bunch more.
I started this post back in 2006 and since then I moved from the original place (NC) where I erected that backstop. It's still there and didn't get much use as the neighbor (brother-in-law) complained that the shooting was causing his two little Pomeranian's to have convulsions.
I have since moved back to the East Coast (VA) and have erected two more back stops where I now live. One is at 100yds in my back yard and landlords cow pasture and the other is in his back pasture across the road. I get a little over 560yds from that one.
Before I used bridge ties and this time I used railroad ties. On the backside of the ties I mounted a 1/4" piece of steel with lag 4" lag bolts. This has been working good as the only thing I've been shooting is Ballistic Tips, V-Max's, a few MK's and Partition's. Calibers have been the .243, .25-06, .260, 270WSM, 7mmRM and .300WM. I didn't have any "dimples" in the metal until last Sundays.
I was doing load development on my .300WM with the 190gn SMK and RL25. Velocities got up to 3100fps. Whew, talk about bone-jarring. I had to consciencely make an effort to close my jaw/mouth before I shot, if not my top and bottom teeth jarred together!
I noticied about mid way through that I had a couple of dimples in the metal. This may be from the ties getting ragged. There seems to be a lot of trains/tracks here in the Shenandoah Valley so ties are a dime a dozen. Good luck. JohnnyK.
I have bunkers at 100,2,3,4,and 600, I called a local excavator and he built them in less than 3 hours cost $375. The 600 yarder is 20 feet long and 14 feet tall.
All the closer ones are 10 by 7. It was worth the money, shouldn't need an upgrade for at least 10 years.
I use big worn out machinery tires, lay them on their side and pack them with sand. Usually two staked on top of each other is plenty high. The best are heavy ply tires like those on Caterpillar wheel loaders. Skidder or tractor tires would probably work well also, this will make the ultimate bullet trap. Most places that have these used tires are more than willing to give them away.
"GOD, GUTS, AND GUNS MADE AMERICA FREE" AND I'M KEEPING ALL THREE!