[MadMax]

Definitely a salient point with gas guns. It appears that AEGs and other spring guns are pretty much constant energy guns in that the muzzle energy is consistent dispite changing pellet weight. However, many gas guns do not exhibit constant energy with different bb mass.

I suspect that when lighter pellets are used, the breech pressure drops because the pellet accelerates down the barrel fast. The rapid expansion of volume behind the pellet results in higher pressure drops in restrictions like in the fire valve or other gas flow paths which causes lower breech pressure.

Reduced breech pressure means less force acting on the pellet (pressure x area) which means less work done accelerating the pellet (reduced force x distance).

This doesn't appear to be the case with AEGs which have very simple gas flow paths. The bore of the nozzle and cylinder centre port is not much smaller than the barrel. The reaction force of accelerating the pellet appears to be the dominant restriction.

Gas guns have more convoluted flow paths and the fire valve takes a little time to fully open. A heavier pellet presents a more significant flow restriction than a lighter one (relative to the flow resistances) so it can absorb more work energy.

In essence, I am conjecturing that an ASG can be approximated as a series of gas flow resistances analogous to electrical resistors in a circuit.

The flow restrictions of various orifaces and the barrel represent a chain of smallish resistances which are constant. They will incur a higher pressure drop at higher flow rates. Similarly a resistor incurs a higher voltage drop at higher currents (Ohms law: V=IR)

However, the pellet represents a wierd resistance. It's resistance varies with time. The pressure drop across a pellet (between breech side and front side) is the difference in breech pressure and atmospheric pressure. The odd thing is the resultant flow rate is dependant on how fast the pellet is moving. A faster pellet ends up having a higher rate of volume change behind the pellet. In a sense, the pellet starts out at a high flow resistance which decays to a lower flow resistance by the time it's screaming out the barrel.

If the constant resistance of the flow path upstream of the pellet is very small, nearly all of the pressure drop occurs across the pellet so different weight pellets will have the same work done on them for the same fixed barrel length ~ constant muzzle energy despite different pellet weights.

If the constant resistance of the upstream flow path is not negligible, then the pressure drop across the various resistances in the flow path will reduce the pressure acting on the pellet so lighter pellets have less work done on them and therefore have higher speed (higher flow rate), but less muzzle energy because the average force acting over the barrel length is less.

Now I have to put my head back together because my brain just exploded.