Author Topic: Tuning 101 (Read 1683 times)

GLPalinkas · « **on:** October 05, 2016, 06:04:00 PM »

Hi Bob, thanks in advance for any help.

When tuning a rifle, what is your starting point?

Do you tune the rifle for the best ES? Shot count?
Do you find the pellet that shoots the best and try to refine the best velocity?

I have followed your advice and throttled my .22 Disco using a reduced transfer point. I tried several I purchased from Crosman, drilled them to different sizes. I settled on .093 because it gave me a 30 shot string with a ES of 16. 2% is awesome, only problem is accuracy went south. South meaning it was shooting better when averages were 850ish. (3/8 ctc 25 yds)

Air Arms Falcons 13.43
Older Discovery with original spring.
Fill 2000-1100
AVG - 707
SD - 4.7
MIN - 699
MAX - 715
ES -16

I don't know what I don't know..... There in lies the problem.

I have re-crowned the barrel, cleaned it with JB Bore Paste, worked a little on the leade by polishing the rough parts and softening the start of the rifling. Added a gen 1 Marauder trigger group, tried different stock Discovery springs cuts to length, pellets inspected, sort for size and weighed.

Again, any help appreciated.

rsterne · « **Reply #1 on:** October 05, 2016, 07:53:07 PM »

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

GLPalinkas · « **Reply #2 on:** October 06, 2016, 03:13:26 PM »

Bob,
Thanks so much for your response. Very logical process. It’s a treasure trove of information and I’m sure a lot of new tuners will appreciate it also.

Sorry for the delay in responding. Hurricane Matthew is coming to visit us in Florida right now. Been getting ready.

A few questions…

1 - You stated …. “I generally start by trying to find out what the maximum power is with the pellet I think I want to use”….. How do you determine the pellet you want to use? Heavier pellet for higher FPE hunting work? Lighter pellet for flatter trajectory while target shooting - FT/BR?

2 - Do certain pellets/weights have an optimum FPS for stability and accuracy? How does one determine what speed a certain pellet likes for the best accuracy?

3 - I agree that the JSB line (Air Arms, also), at least for me, has been the most accurate in my limited experience.
You stated … “I fine tune using the most accurate pellet I have for the intended job.” Would that mean adjust the FPS until maximum accuracy is achieved? Or, does that mean using a different pellet? How do you "fine tune"?

4 - You stated ….“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” ….
I am interested in keeping my FPE below 20 so that I can compete in Hunter FT. That’s why I reduced the transfer port size. However, as I stated, accuracy suffered.

My own intended job would be the absolute best accuracy for FT or BR applications. (Yes, I realize the Benjamin Discovery does not have the most accurate barrel in the world but it’s what I prefer because of lower fill pressures and simplicity.
I fill with scuba tanks and a small CF bottle. I live in South Florida, the scuba capital of the world and it’s just easier and cheaper to get air.) Higher fill pressures of true FT and BR guns complicate things.

Thanks again for sharing your wisdom with everyone on a great site.

Best,

Gary Palinkas
Venice, FL

rsterne · « **Reply #3 on:** October 06, 2016, 06:42:20 PM »

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

GLPalinkas · « **Reply #4 on:** October 07, 2016, 04:21:44 AM »

Again Bob, THANK YOU so much. I didn't even consider barrel oscillation in the equation. The learning continues.(Even at my advanced age...LOL)

I'll adjust and reply back to this thread so others can also benefit.

Gary

maddoghutty · « **Reply #5 on:** December 07, 2017, 11:58:32 AM »

Quote from: rsterne on October 05, 2016, 07:53:07 PM

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.

rsterne · « **Reply #6 on:** December 07, 2017, 01:44:36 PM »

Sorry, I have never used a Huma regulator I suggest you post in the PCP Section.... The regulator (and the pellet consistency) is responsible for the ES.... Most regulated guns are about 1% due to pellet differences, but most are under 2% with unsorted pellets and including the inconsistencies in the regulator.... Generally the hammer spring setting has little to do with the ES, with the exception that if you are tuning on the downslope (ie well below the velocity you can get with your pressure setpoint) you may see larger velocity swings.... just like you see in an unregulated PCP in the first part of the bell curve (the high pressure side).... This is because any slight variation in hammer strike shows up as velocity change when the gun is operating on tiny sips of high pressure (which it does when tuned on the downslope).... It is more efficient, but if everything isn't perfect, has a larger ES....

Bob