A friend of mine, i'll call him Charlie, because thats his name, was caught carrying a 25 auto by some of the gun shop crowd. Every one gave him a good razing, but Charlie felt confident as he had it stoked with the newer "PowerBall" rounds. They have a plastic ballistic tip.
So I conned Charlie to give me a few round to test. I tryed the same test as you did Dave, both with liquid water and ice filled jugs. Never got the bullets to expand. They just dont go fast enough.