What are you laughing at.  E2ss posted a video that makes it apparent what is happening.   Not much different then a fire piston.   A flammable substance is heated by extremely rapid pressure rise and burns creating a second and third cavity.  The bullet has exited the gel completely and could not have ignited the second and third explosions seen in the collapsing cavity.

It's nothing more than childish special effects. The videos are harping on the TC contortions in agonizing and pointless detail.  I couldn't even finish the first video, and I have an attention span that would allow me to watch a feature length film about grass growing.

The only thing that really matters is integrating the total crack lengths of the stationary block and then comparing that to the volume of the visible permanent cavity.

The bullet along with the rapid expansion of the gell caused vaporization within the cavity. Once the cavity rapidly contracted and the pressure inside rapidly increased the vaporized gases ignited.


Pretty simple.

Lucky for you we have a fire protection engineer in the house.

Gypsum is non-flammable, even if you grind it 00 fine.  It's CaSO4.  It will not oxidize under atmospheric conditions.

Yeah, but can jet fuel melt steel beams?

Ah.....NO.  Not all micro fine dust can ignite in the presence of oxygen and heat.  A dust may ignite if the particles are 500 micron or smaller, depending on partical shape, size, composition, air mixing, and ambiant heat or energy input.   Drywall will not burn. It is used as a fire retardant.  It releases water when heated, slowing the heat transfer, and the powder left behind is calcium sulfate...which does not burn.

Fine limestone dust is spread in coal mines to prevent and mitigate coal dust explosions.  

The dust material has to be combustible before it could even be a threat of explosive.

I'm voting for the dieseling theory.  The polymer clear blocks are releasing a fine powder or gas along with trace amounts of carbon, gunpowder.. that are compressed when the temporary cavity collapses

Melt?

No.

Induce thermal stress within load-bearing members leading to fatigue over a period of time?

Yes.

I'm supposed to have super slow motion (~50,000 fps) of an M855A1 round going into clear gel very soon, maybe there will be something in that that can shed some more light.

 

I agree with the posters above that the fact that it seems to burn when the gel contracts means that the compression is playing a part.  

I'm guessing here, but doesn't a bullet ripping through a semi-solid medium at 2300 fps generate a fair bit of heat through friction - if only for a fraction of a millisecond?



 

But what if it was Chinese Drywall, which has Hydrogen Sulfide in it, which is flammable?
Really, though, I stand corrected.  I looked up the OSHA chart, and gypsum is not mentioned.
Vitamin C, however, is...

 

http://en.wikipedia.org/wiki/Sonoluminescence