Well I got all the materials and put together a new backstop today.... The framework is 2x6s with a 3/4" ply back and a 10" x 14" plate of 1/4" steel screwed to that....
The top and bottom rails, fastened to the front, are 2x4s, arranged in such a way that I can slide three layers of 2x4s in without needing to take out any screws.... The front and back layers are 3 pieces each, vertical, and the middle layer is 4 pieces, horizontal.... which left just over 1/2" of space, into which I slid a piece of OSB.... The target clips are taped to that with duct tape....
This means there is 5" of wood before anything can hit the steel plate.... Since all the 2x4 filler blocks are separate pieces, I can replace the center ones, which will get destroyed first, and just turn the side pieces over to put the shot sections out against the edges of the opening.... This should be a lot easier and cheaper than replacing full sheets of OSB.... I guess I'll find out soon how it works, but I'm pretty pleased with it, and I should be able to refill it with about half a 2x4 and a small piece of OSB each time, which will be really cheap to do....
Bob