I probably went overboard on this range, but I didn't ever want there to be doubt about where a bullet went from it.
Here is what I did.
I sank 6X6 posts 3 ft into concrete footings in mostly solid rock(Big fun with a jack hammer to get enough depth).I was concerned because the target boxes are filled with 900 lbs of "washed" (no dirt or small particulates) sand each plus the load from the ties.
The target boxes are 2X12 for the sides and bottom lagged into the posts and 3/4" plywood lagged into the sides for the top and face.I did a test box before I built these that was the same depth and then shot everything I owned at it with nothing penetrating more than 6", including a 300win mag.The pistol bullets stopped in less than 4" and the rifle bullets either disintegrated or penetrated less than 6".I waterproofed the boxes and drilled drain holes in the bottom and put filter cloth over them ,in case some water does gets in.It is very important that the sand stays dry so that it will flow and not pack when the bullet hits it.
On the inside of the target box face ,I stapled old truck mudflaps to retain the sand when the plywood got shot up.This has worked unbelievably well.I have sections in 2 boxes that have 6" holes and the mudflap is still retaining all the sand and barely looks marked.( the nubbins are part of the flap)I will replace 16" square sections of the bad box faces when I get a chance.
The railroad ties are 7"X9" and are stacked 10' high and 16' wide.I toenailed the ties together and to the posts.
This probably would have been enough, but the ties had gaps between them that I was afraid would allow a bullet to sneak thru if someone missed the target box.
So there are semi tires filled with sand behind and sand in between the tires and the ties.I lag bolted the tires together and to the ties and then foamed anyplace that leaked sand.I will finish the tire stack now that I have the tractor back at the place and heal from my most recent misadventure.(close encounter with a bandsaw.OUCH!!!!!!)It would have to be my trigger finger , it makes typing fun too.