I am not entirely sure what is happening with this gel block test.

It is pretty standard stuff as the bullet penetrates and rips through the gel medium. You can see the temporary wound channel. However as the gel block contracts to its normal shape, there is a micro explosion inside the gel block. Which is followed by gases escaping as a form of flatulence out the entry wound. But what caused this micro explosion? Could it be unburnt gases traveling inside the wound channel? If that is the case how is there enough heat and air/fuel mixture for the detonation? Anyone have any good ideas about this phenomenon?