Dear Mikey,
You are right. The way I fixed it was to get a large sheet of steel, but only 3/8ths inches thick, slanted down at a 45% angle, with sand below. All of the bullets go into the sand with no problem, even from a rifle. I think the angle is almost more important than the thickness of the steel. I also took a sheet of 3/4inch exterior grade plywood, stained with exterior stain, and just leaned it back against the wall of railroad ties at a slight angle (almost flat up against the wall, but not quite). In case a bullet escapes or misses the steel plate, it will go through the plywood and hit the ties, but any backward ricochet bullet will then come backward and hit the plywood, which will give a little, and the bullet does not make it back through the plywood. It is trapped.
Also, my railroad ties were not old, used, wornout ties full of cracks and soft places. They were brand spanking new, and very deeply and thoroughly soaked with the tar/creosote preservative. I bought them from a company that sells them to railroads. There is no metal in them. I can only guess that they are hardwood? At any rate, as I said, after baking in the sun, they are like steel. You can't even hammer a nail into them. After about one-half of an inch into the tie, the nail just bends. (Even a 16 penny nail.)
Best Regards,
Big Paulie