CRS,
Haven't heard of any formal studies involving bullet expansion in different twist rates, but would imagine ballistic gelatin or something like the Test Tube would provide some info.
One of the interesting things about Test Tubes (which were made of relatively soft candle wax) is that unlike ballistic gel they left a permanent wound channel. When sectioned, you could actually see the marks left by the bullet as it twisted through the wax. With lead-cored bullets this normally matched the rifling twist, with one turn in X inches, but with petal-type bullets the rate of turning increased as the bullet penetrated.
Some people (including me) guessed that the increased rate-of-turn was due to the slight propeller angle of the expanded petals, which "grabbed" the wax. Others suggested it was simply due to the bullet slowing down, while it continued to turn at the same rate. I would then ask, "Why doesn't the same thing happen with lead-cored bullets that don't expand in the typical petal shape?"
But I suspect the best test would be to shoot a lot of animals, and record as much data as possible. It wouldn't surprise me that some of the effectiveness of 6.5 rounds (whether the old-timers like the 6.5x55 or newer cases) is due to the typical twist of 1-8 or so.
