The effect of the media on the bullet Of course, the human body is not made of a consistent substance all the way through, and it is almost impossible to devise a test media that can simulate the effects of a bullet as it passes through skin, soft tissue, muscle, bones, or spongy tissue. However, using a tissue simulant will allow one to compare the relative performance of one load to another. The first problem with this test media is that it can vary significantly in texture, moisture content, and density. It also may or may not have air pockets. This makes it impossible to obtain repeatable results.
Bullets do expand in clay, but expansion is exaggerated.
A temporary cavity with a shape consistent with reality is formed, but the dimensions are greatly exaggerated. The permanent cavity's depth of penetration is greatly under-exaggerated. In fact, due to the fact that clay is not elastic, the effects of the temporary cavity are easy to see and photograph, but that is about all that's useful about clay as a test medium. Summary, clay grossly over-exaggerates the effectiveness of a bullet, and should be avoided. Some people use newsprint, others prefer telephone books.