In early July 2015 a few bullets were shot into water bottles, and captured, and looked at. Powder charge was 50g Blackhorn, and range was 25 yard. All the bullets expanded, and mushroomed; worked good. Grouse45 suggested we put something in front of the bottles to see if different bullets reacted differently . Concrete, and other things were suggested. Wife, and i discussed what she had laying around he house, which could be used in front of the water bottles. She found some shelving, and some carpet scraps. The old shelving was 3/8" thick.
We put the junk carpet, and shelving into use in front of the water bottles. Some bullets worked better than others. Some thought we should use deer hide, and deer bone, but we didn't, because we wanted to use materials that would be consistent/identical, and a material we could readily find. Basically the reason for the plywood, and carpet is to separate different bullet results from each other. All bullets were/are shot through the same media.
Does plywood kinda sorta resemble rib bone?