Lets take my .25 cal marauder for example, it sports a 19.5" barrel, .225" ports and 53 cc plenum @ 2100~ psi. All the graphs shown are with 33.9 gr.
What happens with various barrel lengths at plateau power? Well for one, if you chop the Marauder barrel in half, it will output basically half the energy, if you double the barrel, it would nearly double the energy, with the right weight projectile, or if you add 10% barrel length, you will add nearly 10% energy potential to the system. This wouldn't come for free, and you would need to dwell and keep the valve open nearly twice as long to get twice the air into a system with twice the barrel of course to obtain twice the energy...
So when someone asks, how many fps will 1" of barrel add? Well that depends on your existing barrel length! For a 10" barrel, 10% FPE, for a 20" barrel, 5% FPE, then you can determine your velocity/fps increase based on that. Clearly adding 1" to a 6" barrel increases FPS a lot more than adding 1" to a 28" barrel.
What happens with various port sizes? Well if I half my port size from .225 down to .1125, I would get half the power at plateau...same applies in reverse, you double power output. 10% increase/decrease should relate closely to a 10% change in energy. Likewise you would use more air as power increases, or less as it decreases at plateau as you're expending more or less appreciable air behind the pellet before it exits the muzzle, provided you adjust your hammer strike. Largest port possible closest to bore diameter allows the shortest dwell time possible in a system as the lift generated to expel for example 1000 mg of air through a .25" equal port for a .25 cal takes less time than that of a .2" equal port or less, thereby increasing dwell time / the pressure pulse making it less efficient than a system with larger porting closer to bore diameter.
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Half the pressure? Half the power!
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All this to say, energy increase is fairly linear with Pressure, Barrel Length changes, and Port sizing changes until you reach limitations placed by physics, be it speed of sound, or otherwise, such as exceeding the diameter of bore on air flow path, or exceeding regulator setting/fill pressure. However doubling the caliber would provide closer to 4x the power increase if you're on the hunt for power.
And projectile weight where power stops increasing once pellet weight is 1/2 the FPE in grains and velocity is 950~ fps. This can be seen middle of chart.
And of course, plenum changing alone...
Where the graph goes off the chart its flat and shows no real gains to be had, beyond 3 fpe/ci shows no real gains for a pcp's air delivery system, however a good sweet spot for max power is a bit over 1 fpe/ci with a good workable range being .4 fpe/ci to 1.6 fpe/ci. One could supplement smaller plenum volumes with increases pressure by that is not the point of this discussion as one could simply run longer barrels as well, or larger ports, it all provides more energy if plenum volume won't get you there, unless you want to max everything out, then you should be at your max regulator setting anyhow and that discussion is moot. Hope this helps some tinkers out there understand changes they can or have made to their airguns.
Regulator refresh rate? Oh ya, theres that too...it effects your pressure during the shot cycle!
-Matt
What happens with various barrel lengths at plateau power? Well for one, if you chop the Marauder barrel in half, it will output basically half the energy, if you double the barrel, it would nearly double the energy, with the right weight projectile, or if you add 10% barrel length, you will add nearly 10% energy potential to the system. This wouldn't come for free, and you would need to dwell and keep the valve open nearly twice as long to get twice the air into a system with twice the barrel of course to obtain twice the energy...
So when someone asks, how many fps will 1" of barrel add? Well that depends on your existing barrel length! For a 10" barrel, 10% FPE, for a 20" barrel, 5% FPE, then you can determine your velocity/fps increase based on that. Clearly adding 1" to a 6" barrel increases FPS a lot more than adding 1" to a 28" barrel.
What happens with various port sizes? Well if I half my port size from .225 down to .1125, I would get half the power at plateau...same applies in reverse, you double power output. 10% increase/decrease should relate closely to a 10% change in energy. Likewise you would use more air as power increases, or less as it decreases at plateau as you're expending more or less appreciable air behind the pellet before it exits the muzzle, provided you adjust your hammer strike. Largest port possible closest to bore diameter allows the shortest dwell time possible in a system as the lift generated to expel for example 1000 mg of air through a .25" equal port for a .25 cal takes less time than that of a .2" equal port or less, thereby increasing dwell time / the pressure pulse making it less efficient than a system with larger porting closer to bore diameter.
Half the pressure? Half the power!
All this to say, energy increase is fairly linear with Pressure, Barrel Length changes, and Port sizing changes until you reach limitations placed by physics, be it speed of sound, or otherwise, such as exceeding the diameter of bore on air flow path, or exceeding regulator setting/fill pressure. However doubling the caliber would provide closer to 4x the power increase if you're on the hunt for power.
And projectile weight where power stops increasing once pellet weight is 1/2 the FPE in grains and velocity is 950~ fps. This can be seen middle of chart.
And of course, plenum changing alone...
Where the graph goes off the chart its flat and shows no real gains to be had, beyond 3 fpe/ci shows no real gains for a pcp's air delivery system, however a good sweet spot for max power is a bit over 1 fpe/ci with a good workable range being .4 fpe/ci to 1.6 fpe/ci. One could supplement smaller plenum volumes with increases pressure by that is not the point of this discussion as one could simply run longer barrels as well, or larger ports, it all provides more energy if plenum volume won't get you there, unless you want to max everything out, then you should be at your max regulator setting anyhow and that discussion is moot. Hope this helps some tinkers out there understand changes they can or have made to their airguns.
Regulator refresh rate? Oh ya, theres that too...it effects your pressure during the shot cycle!
-Matt
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