This is M855A1 being fired into clear ballistic gelatin. The shot was taken from about 10 feet using an AR15 with a 16" barrel. View 1 is 14,300 FPS and View 2 is 30,000 FPS. Thank you to http://www.aimed-research.com/
for helping out with the camera work. Thanks to http://clearballistics.com/ for providing the gel block! The flash/explosion in the gel hasn't been very well explained so far. The best explanation I have is that the hot bullet vaporizes some of the gel (which is flammable) and between the friction, heat of the bullet, and air being sucked into the temporary stretch cavity, as the TSC collapses it acts like a diesel engine (or flame piston) and compresses the mixture of heated gel vapor and air until it explodes. You can see the exhaust gas exiting the entrance hole.