Hate to disagree but after running an indoor range for ten years can I say that 3mm is not overkill unless you are only shooting 6ft.lb.

We've moved away from 3mm in some cases as constant impact in one place by 177 and 22 pellets from 12ft.lb rifles bends it. Our LSR boxes (20 yards range) are now being fitted with 5mm backplates.

We made a number of boxes from 12mm ply sides and a steel backplate. 177 pellets hit the backplate and shatter into tiny shards of lead which fly off sideways at roughly 90 degrees. These shards have eroded the 12mm ply and in one case we found last month, only one lamination of ply to go before there was a hole in the side of the box.