Thanks for the great ideas folks. I think the combination of the homemade ballistics gel and the soaked leather will make a good combination of media. So I can get a picture of the wound cavity through the gel, and something a little more resistant to stop the bullet but not too hard to mess with deformation, like the leather.
If you really want a good prediction of what your bullets should do, plan to test them in several different test mediums, and compare to some known good standard, like a Nosler Partition for example. If you don't have a good standard to compare to, the test is pretty meaningless.
At a minimum I like to do separate tests in water jugs, wet paper, dry paper, and some sort of bone if possible in front or inside the wet mediums. All of those tell you different things about what the bullet can do. York's wet leather scraps sound good too, although I haven't tried that myself. Gel is good if tightly controlled, but can give you wide variations in results if you let temperature and consistency fluctuate.
For testing defense (anti-people) rounds, I like to throw in some barriers too, like car door sheet metal, glass, denim, etc as those things also have an effect on performance.
