I know you may not want to hear this, but grab a shovel!

That is unless you have access to a bobcat or dozer.
Nothin' stops a bullet like good ol' dirt, but be sure to throw most of the rocks out to help curb ricochets. I've got dirt backstops at the 100, 125, and the 150 yard lines at my range, with no problems at all.

Good luck with whatever you decide!
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Screw the Anti's!!