What is airgun efficiency? It's calculated based on shot count obtained within a specified pressure range at the average energy over said pressure range. How can we manipulate efficiency?
Efficiency is directly tied to many things, from barrel length, to plenum volume, however the most crucial, and the one I'll discuss briefly is port diameter. Efficiency in itself is a non-linear bell curve, meaning at plateau (peak obtainable fps) it'll be at or near its lowest efficiency, and as you detune / reduce the power of an airgun, the efficiency will climb, that is until you reduce power so much, efficiency will stagnate and then decline due to other losses overcoming the reduction of air being released, primarily, projectile to bore friction, and projectile break away friction.
The further from plateau (peak fps) you tune a gun, the more efficient on air it becomes. Plateau is decided by many factors, but the critical one being discussed which can easily be manipulated via a velocity metering screw, a quarter turn valve, a drill bit, or rotary tool, is the port diameter.
This means, that a .22 cal ported to 70% of bore @ .154" will be less efficient at ANY equal or less power level compared to another otherwise equal .22 cal ported to 85% of the bore @ .187", however, if tuned to its plateau, the 85% bore ported .22 cal will begin to consume more air, as its Mass Flow Rate increases and loss always outpaces gains in efficiency the more energy you attempt to extract from a given airgun.
This does NOT mean your average user should go full bore porting, or even close, without extensive knowledge of an airguns limitations, from loading a pellet without causing damage, to what other obstruction in the system may limit the port diameter equivalency at that particular section of airway.
The caveat to de-tuning further and further from your plateau, is that fps from shot to shot (extreme spread) will likely increase without some sort of mitigation (valve lift limiter, and valve balancing via closing force/opening force ie valve spring/hammer spring balance), due primarily to variance in reg pressure from shot to shot, be it 1-2 bar or even .5 bar, from minor shifts in the energy produced by your hammer strike be it from surface quality, angle of gun, and even sear disengagement inconsistency.
There are two ways (that I am aware of) to calculate efficiency, thermal, and volumetric, however if you were to graph both, the change in efficiency would appear identical. So identical structure, just different numbers which represent said structure, or plots.
Volumetric efficiency is commonly expressed as fpe/ci (or for some, joules/cc), where as thermal efficiency is simply seen as a % value of energy obtained from the overall energy released. Most airguns operate around 25-30% thermal efficiency when tuned well, which translates to approximately 1.3-1.6 fpe/ci.
However, when tuned to absolute plateau, or peak fps, the efficiency is generally .9-1 fpe/ci, or 15-18% thermal efficiency. The difference in power from a 1 fpe/ci tune compared to a 1.3 fpe/ci tune is marginal, between 5-10%, however the difference is efficiency in 30%, meaning you only have to sacrifice around 5% of your plateau to potentially increase your shot count by upwards of 30%. This not only increases efficiency, but also tames the shot cycle, as you will notice less recoil due less air being lost when exiting your muzzle...as well as less overall energy being initially released into the bore.
This means that, if you plateau at 35 fpe, you're ejecting around 233 FPE through your valve, however, most of this energy is lost thermally before the projectile absorbs it, and the remainder is ejected from the barrel as the pellet outpaces it. Insane right? Even when tuned efficiently at 30% thermally, you are losing around 82 fpe and ejecting a total of 117.
The difference in 117 fpe being released from an airgun valve and 233 is, a lot, and there is without a shred of doubt, every single shooter will notice the difference between the two tunes.
Food for thought. Hope you enjoyed the read and have yourself a great day!
-Matt
www.airgunnation.com 
