Here's a viedo showing the Bushmaster hitting ballistic gel. It looks like expansion is pretty quick and at a perfect penetration depth for deer. The video says it's "over 2000 fps", but not how much over. I'd sure like to know the actual muzzle velocity of the round shown in the video, and the distance of the gel from the muzzle.
http://www.youtube.com/watch?v=sC0f5wvgGew