The AirGun Guild

A friendly place to share one's interest in all things airguns.

I generally start by trying to find out what the maximum power is with the pellet I think I want to use, and how far I can back off the hammer spring preload before the velocity starts to drop (the "knee" of the velocity vs. preload curve).... This is only because I am curious, and have a tendency to tune for about as much power as I can obtain efficiently.... If you have a given power level (FPE) in mind, that part of the process is a waste of time (although you may learn something, never a bad thing)....

If I am tuning for power, using a given fill pressure and pellet weight, I start at the "knee" (no sense going higher, its very wasteful of air, and results in a Korean Cliff style of tune).... I then back off the preload until I get a typical bell-curve, where the first half of the curve rises 4% of the velocity at the peak, or slightly less, and stop shooting when the velocity falls back 4% below the peak.... This gives me a 4% ES, starting at my fill pressure, and giving the most shots I can get within that ES.... Since a 4% ES is good to about 50 yards, that is generally where I stop, as that covers most of the shooting I would do with an unregulated PCP.... If I happen to be tuning an unregulated PCP for longer range, I use a tighter ES.... 3% for 75 yards, and 2% maximum for 100 yards....

With a regulated gun, I expect less than a 2% ES always, if I don't get it, there is something wrong.... Usually the ES with unsorted pellets is a whisker over 1%.... Since I seldom shoot over 100 yards, tuning a regulated gun becomes a matter of adjusting regulator setpoint pressure and hammer spring preload until I get the FPE level I want with the greatest shot count.... This inevitably occurs when the gun is tuned just below the "knee" of the curve, ie below the plateau and above the downslope, or right at the top of the downslope.... Any major change in pellet weight will change the tune, of course, whether regulated or not....

After I have achieved the tune I want, I then double-check all suitable pellets.... If I didn't guess right the first time, and find something else is more accurate.... I fine tune using the most accurate pellet I have for the intended job.... 95% of the time that ends up being some weight of JSB Exact....

If I want significantly less power than the gun is capable of, then I either back off the hammer spring preload and tune for a lower fill pressure (or regulator setpoint).... or install a smaller transfer port.... or both.... Chokng up on the transfer port allows you to maintain the original fill pressure while reducing the FPE level, something you can't do with hammer spring preload.... because less power is then accompanied by lower operating pressures....

Bob

Answers by the numbers....

1. For almost all my airguns, I want to shoot in the mid 900s if possible, for the best balance between trajectory and wind drift.... Least drift actually occurs at a slightly lower velocity, about 800-850 fps for most round nosed pellets, so you might want to reduce that slightly for your use because trajectory is predictable, the wind is not.... Based on the power I have available I then grab the weight of JSB that will give me the velocity I want and start there.... For 20 FPE in a .177, that would be the 10.3 gr Exact Heavy at about 900 fps....

2. See above for my starting point.... Fine tuning for best accuracy is usually done by small adjustment to the velocity.... It can also be done with a barrel tuner.... In both cases, you are trying to get the pellet to reach the muzzle near the top of an oscillation (ideal), or next best, near the bottom....

3. Since I start with the JSB, the only time I would have to change pellet is if something else is obviously more accurate around my chosen FPE level in that barrel.... After that, see #2 above....

4. It is most likely that your new velocity is one where the pellet is exiting the muzzle in the middle of a barrel oscillation, where the muzzle is moving the fastest.... In that case, any tiny variation is emphacized.... at a slightly higher or lower velocity, the barrel will pause briefly at the end of its arc as it changes direction.... leading to the tightest groups....

Hope that helps....

Bob

I generally start by trying to find out what the maximum power is with the pellet I think I want to use, and how far I can back off the hammer spring preload before the velocity starts to drop (the "knee" of the velocity vs. preload curve).... This is only because I am curious, and have a tendency to tune for about as much power as I can obtain efficiently.... If you have a given power level (FPE) in mind, that part of the process is a waste of time (although you may learn something, never a bad thing)....

If I am tuning for power, using a given fill pressure and pellet weight, I start at the "knee" (no sense going higher, its very wasteful of air, and results in a Korean Cliff style of tune).... I then back off the preload until I get a typical bell-curve, where the first half of the curve rises 4% of the velocity at the peak, or slightly less, and stop shooting when the velocity falls back 4% below the peak.... This gives me a 4% ES, starting at my fill pressure, and giving the most shots I can get within that ES.... Since a 4% ES is good to about 50 yards, that is generally where I stop, as that covers most of the shooting I would do with an unregulated PCP.... If I happen to be tuning an unregulated PCP for longer range, I use a tighter ES.... 3% for 75 yards, and 2% maximum for 100 yards....

With a regulated gun, I expect less than a 2% ES always, if I don't get it, there is something wrong.... Usually the ES with unsorted pellets is a whisker over 1%.... Since I seldom shoot over 100 yards, tuning a regulated gun becomes a matter of adjusting regulator setpoint pressure and hammer spring preload until I get the FPE level I want with the greatest shot count.... This inevitably occurs when the gun is tuned just below the "knee" of the curve, ie below the plateau and above the downslope, or right at the top of the downslope.... Any major change in pellet weight will change the tune, of course, whether regulated or not....

After I have achieved the tune I want, I then double-check all suitable pellets.... If I didn't guess right the first time, and find something else is more accurate.... I fine tune using the most accurate pellet I have for the intended job.... 95% of the time that ends up being some weight of JSB Exact....

If I want significantly less power than the gun is capable of, then I either back off the hammer spring preload and tune for a lower fill pressure (or regulator setpoint).... or install a smaller transfer port.... or both.... Chokng up on the transfer port allows you to maintain the original fill pressure while reducing the FPE level, something you can't do with hammer spring preload.... because less power is then accompanied by lower operating pressures....

Bob

Bob after reading your answer on the topic of tuning a pcp I posted one this week and this has the answers I'm after but would like to know one thing I'm frustrated about is, I purchased my huma reg already preset for my sub 12ftlbs rifle so do I just adjust the hammer spring preload to try and get the 1% es. Or would I have to adjust the reg also. I would like to ask Bob if I do have to adjust the reg also which way do I turn the reg screw. I presume it would be anti clockwise to drop in pressure due to the 12ftlbs setting or would it be clockwise for more pressure. That is the two answers I'm stuck on to get the smallest es on my rifle I can I will look forward to your reply thx.