Ethanol/Oxygen Cannon

The cannon was filled with oxygen and a very small volume (less than 1 cm3) of ethanol (C2H5OH). This was shaken just before the demonstration in order to evaporate some of the ethanol. The mixture was set off with the spark from a piezoelectric ignitor. The bung travelled around 30 metres.
Ethanol is the active component in alcoholic drinks. Many people don't realise how much energy ('calories') it contains. The energy from this cannon came from less than 1 cm3 of ethanol.
At around 00:00:39 you can see the remaining ethanol in the cannon evaporating from the hot barrel after the explosion.
Watch the slow motion clip carefully at around 00:01:29. There is a frame where the barrel appears to elongate just before the bung departed! What do you think happened? It could be a distortion of the barrel. This could also be an illusion, caused by the speed of the recoil or by the refractive index of the air being affected by the intense heat of the explosion.