A hot bubble inside of a liquid caused by cavitating gasses, funny things happen when you watch high speed footage of .44 Mag and ballistics gel in action.
Sonoluminescence is a a phenomenon that occurs when gas bubbles are suspended and driven in a liquid at ultrasonic frequencies. This results in bubble collapse, cavitation, and light emission. According to this site the process was first observed in 1934 but it took a while (until 1988) for physicists to figure out exactly what made the bubbles glow.
However, through the magic of ballistics gel and high speed cameras, gun geeks get to be in on the science, say when you fire a S&W 4″ 629, plugging a Barnes copper 225 grain XPB hollow point into a block of gel 7 yards away while rolling at 40,000 frames per second in the excellent vid from Bullet Theory above.