Daryl, what you found, by your experiments, is predictable.
Every time you work copper, it gets harder, hence the experience of the drawn bullets penetrating further and as you say, if they loose their cores in the game animal.....doesn't matter much, so long as they kill it.

I have resized FMJ .303 bullets to shoot in my .308, but they were only for target use and couldn't have come apart anyway.