Just had to watch that a bunch of times.

“…and would blow your head clean off, you gotta ask yourself…”

Now what the heck is causing that second flash, within the gel block? Is the gel igniting under some kind of ultra-high rebounding compression that has compressed the air cavity?

Aggie – I was wondering that myself. The gel is locally heated as energy is transferred from the bullet. In addition, there are lots of shock waves bouncing around in that block and the hit appears on axis so the opportunity exists for shock convergence. You can also see a black residue in the region where the flash occurred. I suspect some of the gel burned. The smoke also provides evidence that a reaction occurred. I don’t know much about gel so take these comments with a grain of salt.

Two words: Implosive Cavitation. Kojak sez, Look it up, baby.