...the only (very simple) idea that occurs to me at the moment to add to your already excellent explanation that would help account to some "degree" (no pun
) for the observed effect of the HDD increasing muzzle energy is...
1. The tendency of CO2
vapor pressure - and therefore power - to decline with cooling is very well known.
2. Every time a CO2
powered gun is fired, more CO2
liquid must evaporate to replace the expended gas, which consumes heat and therefore tends to chill the gun, lower pressure, and reduce power.
3. Consequently the efficiency boost provided by an HDD, which means less gas is expended (wasted) with each shot and therefore less evaporation, heat loss, and cooling, should translate to a warmer gun, more pressure, and more power.