Reply To: thickness of the steel sheet to stop pellets
+1 on the rubber mulch to capture the pellets. A bag is cheap ($7-$10), it’s quiet, safe, and easy. In my testing from 1 foot away, 7″ of it stopped 20 consecutive shots (in the same spot) from a .22 springer putting out 22 FPE. For insurance, I use a piece of 1/2 plywood. Yes, the plywood may not stop the pellet by itself, but shouldn’t have to – it’s there just in case something actually makes it through the box (which has yet to happen to me). 1/8″ steel should be more than adequate if that’s the route you want to go.
Rubber mulch can be in chips or shredded. You want the chips – they move more freely, thus are better at “filling the hole”.
Fill the box to the very top and shake it to settle the chips. Keep adding as they settle and make sure that it’s filled right to the very top when you close the box – you want the box to be 100% full. Even when you do this, the top 2-3 inches of the box will not be as good at stopping pellets.