......why the plywood and carpet? A piece of raw hide would seem to be better for simulating hide and plywood has little in common with ribs.
Because they were readily available, and are consistent. The object in these tests was to compare bullets. The 3/8" plywood was used in every test. Carpet was used in every test. All tests were done using the same exact type water jugs. Range to the carpet was the same in every test. The powder charge was exactly 80g, or 50g Blackhorn, in every test.
Hide or bone wouldn't have been consistent. In these tests, all the bullets saw the same.... Some failed, some worked. Failed bullets failed the same test that successful bullets passed. Remarkably, the bullets tested in this fashion, performed the same on animals as they did in the testing, whenever a comparison could be made.