http://bentasker.i2p/posts/blog/house-stuff/building-an-air-rifle-target-backstop.html
If the pellet manages to pass through those, it then reaches the much harder 9mm plywood. A sheet of steel could achieve a similar effect, but the aim of the backboard isn't just to stop the pellet, it's designed to do so as quietly as possible: I have neighbours, so flinging lumps of lead at a noisy backboard is likely to be unwelcome.