Heres a few I dug outta a old dead tree. Along with maxi-balls and round ball. I cut them all in half with a knife... I had to wear a leather glove, pad the back of the knife, and put all my weight on it to cut the ball-ets. They are surely harder than roundball. Not a WHOLE lot, but harder.
True, a tree isnt like flesh. In fact, my grandpa shot a deer once, we went to the truck to get knife, pen, and take off some clothes. When we got back he was sitting up, looking around. In an effort to not lose meat, I shot it again in the neck with a PRB. RB followed its spine, ended up in the opposite side, just under the skin, flattened out at least as big as a quarter.
OTOH, I've found some that barely expanded. I reckon it all has to do with wether they went though the chest, hitting soft lungs, or someplace else, hitting lots of bone and such.
Anyway, the tree still shows that the ball-et probably wont expand as much as a roundball. All of them were out of a .50
EDIT: For a bit bigger pic, click
here I wasnt aware the forum re-sizes them.