Airgun guru Bob Sterne previously described a “lofty goal” for PCP design to obtain 50% of theoretical maximum power for a given caliber, barrel length and pressure:
hardairmagazine.com
As Bob mentions, 50% of theoretical power is a “lofty” goal because much power is lost to air propulsion, cooling and expansion, in addition to the friction of the projectile in the barrel. Hence, very few PCPs reach this goal and they tend to be specialist builds. Bob writes:
“Well, even the best custom PCPs have trouble reaching even 50% of the theoretical maximum power from a PCP air rifle for a single, maximum power shot, when using air. ”
And:
“I must admit to never quite achieving my “lofty goal” when using a bullet traveling at 950 fps or above, which is usually my target velocity. I can get to about 90% of my goal, for a single shot at maximum power, and other skilled builders have had similar results.”
For fun, knowing that it can be a very powerful beast, I decided to see how the GK1 fares in this context: I’m using data from @JDGriz, who ran a string from the .25 GK1 using the Altaros 50grain slug and recorded pressures:
www.airgunnation.com
JDGriz measured 88fpe at the GK1’s full power setting for the slug with a starting pressure of 350bar and a pressure after the shot of 330bar. So I’m going to say that the median pressure of the shot is 340bar, or 4931psi.
Now, if we use Bob’s lofty goal formula for .25 with an 8.2in barrel, the “lofty goal” (50% of theoretical efficiency) FPE is 82.7fpe. I’ll spare you the calculation but feel free to check it.
So, the GK1 handily exceeds Bob’s lofty goal, of 50% and actually makes 53% of maximum theoretical power.
Then, if we use JDGriz’s data for shots taken at a lower reg pressure, where there’s likely a little less air wasted, we find that the GK1 is making 56% of theoretical power!
Based on this analysis, it seems that the GK1, even with a high friction slug, is pushing past what can be achieved using a conventional PCP with a hammer.
While I appreciate the fact that its revolver action (shooting from a mag) is not optimal for accuracy, I’d love to see this valve used with a (perhaps retractable) bolt and probe to produce what could be a very accurate and powerful PCP in a small form factor. In any case, the valve is clearly groundbreaking!
How To Obtain The Maximum Power from a PCP Air Rifle
This time, we’ll discuss how to obtain the maximum power from a PCP air rifle, together with factors that limit that power.
hardairmagazine.com As Bob mentions, 50% of theoretical power is a “lofty” goal because much power is lost to air propulsion, cooling and expansion, in addition to the friction of the projectile in the barrel. Hence, very few PCPs reach this goal and they tend to be specialist builds. Bob writes:
“Well, even the best custom PCPs have trouble reaching even 50% of the theoretical maximum power from a PCP air rifle for a single, maximum power shot, when using air. ”
And:
“I must admit to never quite achieving my “lofty goal” when using a bullet traveling at 950 fps or above, which is usually my target velocity. I can get to about 90% of my goal, for a single shot at maximum power, and other skilled builders have had similar results.”
For fun, knowing that it can be a very powerful beast, I decided to see how the GK1 fares in this context: I’m using data from @JDGriz, who ran a string from the .25 GK1 using the Altaros 50grain slug and recorded pressures:
Huben - GK1 .25 tuning for Altaros & JSB - charts and data
I've compiled some chronograph data for the GK1 V3 in .25, with Altaros 50gr slugs. One of the data columns is with 51gr slugs, because I ran out of 50s (they are smooth-walled and this might be reducing the power by 2-3 fpe, looking at the data). Unscrewing the power screw all the way until it...
www.airgunnation.com JDGriz measured 88fpe at the GK1’s full power setting for the slug with a starting pressure of 350bar and a pressure after the shot of 330bar. So I’m going to say that the median pressure of the shot is 340bar, or 4931psi.
Now, if we use Bob’s lofty goal formula for .25 with an 8.2in barrel, the “lofty goal” (50% of theoretical efficiency) FPE is 82.7fpe. I’ll spare you the calculation but feel free to check it.
So, the GK1 handily exceeds Bob’s lofty goal, of 50% and actually makes 53% of maximum theoretical power.
Then, if we use JDGriz’s data for shots taken at a lower reg pressure, where there’s likely a little less air wasted, we find that the GK1 is making 56% of theoretical power!
Based on this analysis, it seems that the GK1, even with a high friction slug, is pushing past what can be achieved using a conventional PCP with a hammer.
While I appreciate the fact that its revolver action (shooting from a mag) is not optimal for accuracy, I’d love to see this valve used with a (perhaps retractable) bolt and probe to produce what could be a very accurate and powerful PCP in a small form factor. In any case, the valve is clearly groundbreaking!
