I didn’t see the “illus 26 above” in your posting, but I have seen the .223 round go through the gelatin block and similar things happen. I don’t know what sets off the explosive action but I do know it travels for some distance and then explodes.
It’s not so much that it is a “carcano” round, it’s that it is a metal jacked bullet designed to go through more than one soldier on the battle field. One time my brother and I were out shooting and we had an old army masser WWII vintage, not sure where it was from but it had metal jacked bullets in it. I fired at some sort of animal feed bin and the bullet went through the first 2X6 wood piece and when it went through the second wood rail, it went through it sideways, they were about 5 feet apart. So in the first piece of wood there was just a round hole in it and in the second one you could see the side profile of the bullet. No evidence of it coming apart.
I will look in McLarens book and see if there is a moment where McLaren or Donohue realize that they don’t have an acceptable representative experiment, no hard shell.
I haven’t seen anything about the 6.5 going through a skull but McLaren did do the watermelon test with both types of bullets. The frangible round, .223 acted exactly as in Dr Alvarez’s experiment, small entry hole and an explosion in the exit area. The 6.5 had a small entry hole and the exit hole was a little larger with some outward cracks around it. It was in “JFK – The Smoking Gun” documentary. Again a watermelon is not a human skull but it does give us a very good idea of what is happening.