I was visiting with a new customer today who happens to manufacture all things for indoor shooting ranges: bullet traps, air handling, target carriers, etc. After our meeting I asked for tour, and they showed us how the traps are made. This is a side view of the trap:
At the trap end of the range, the "floor" and "ceiling" are made of of super hardened steel (these guys are the second largest user of this type of steel. US Navy is the largest). They come together to make a funnel that leads to the trap - in this picture the bullet would enter in the top right, go round and round counter clockwise, then fall out the bottom. I've always wondered how these work. I thought it was pretty neat. They also manufacture all sorts of portable reactive steel targets as well as full blown "houses" for military and LEO training. The house has reinforced walls and catwalks above for the instructors.
Lane