sniper: The difference between the cat and the steel can be best described as hydraulic or hydrostatic action in the cat, and pure kinetic energy displacement at a single point on the steel.
With the cat, the bullet expends it's engergy in a hydraulic medium in all directions. Pascals Law states that in a liquid enviromnent, all pressure imparted is spread equally in all directions (you cannot compress water). With the steel, the entire energy displacement and momentum is centered/massed on one point, and the full energy potential is used to push directly through the steel, causing complete penetration.
Just another example of just how powerful a varmint caliber really is, and a very good example at that.
On a side note: When during the war I was witness to what the .50 BMG round would do to various materials, including mild steel and armor, it was the biggest wake-up call I've had in this regard, as I'd been around rifles my entire life right up to that time. Your demonstration above would be good to show young shooters to illustrate the pure power of a rifle......three words come to mind immediately: power, respect, and safety.....
So.. . I find it interesting that the round can penetrate steel yet not exit a kitty. Once again, I am amazed at the calibre called the .204! Kudos to the wildcatters 
