Thread: Artificial sighting in backstop

If I was to build an artificial backstop at 100 Mtrs for siting in and use say six strainers posts then all walled in with 6 x 2 timber so a box then filled with sand anyone know how wide the box should be to prevent any shots exiting the rear wall.
I was thinking 1.8 high by 2.0 long and unsure of required width .I d just fill the sand part with tractor bucket
Up to 300 win mag calibre ??
Cheers