Our frame rate for this video was 40,000 frames per second. Distance from the muzzle to the gel block was 7 yards. Revolver is a standard S&W 4" 629, firing a Barnes copper 225 grain XPB hollow point. We didn't chrono the round but I would guess that the muzzle velocity was hovering around 1150 fps.
The fireball in the middle of the gel block is a phenomenon that we observe regularly at these frame rates. our best guess is that it is an example of the sonoluminescence effect.