Chopped up tyre does work but make sure you allow for fire. Especially if some dolt uses tracer etc. For testing set up a board wtith packed sand behind it and shoot it up close and dig out the bullet to find out how far it travelled. Then add an extra buffer in case you get a bigger calibre later and go from there.
Finally, make sure that whatever is behind the backstop is a clear firing zone (IE nothing there that could be damaged by a stray etc. ) Better to be paranoid and not need it than wrong.....
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