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Tuning The high reg "short burst of high pressure" tune

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Khornet

Khornet

Member
Feb 10, 2020
194
5
NH, United States
Can someone explain this process? I upped my reg from 95 to 125 or so, but I don't want 995 fps, which is my max at that set point with 23 gr H&N .218 slugs. Now do I reduce hammer preload using the screw to get 97% of max velocity, then close the valve adjuster til I get my target speed of about 930? Or do I leave the hammer at max and just close the valve? I've tried both and am getting wide velocity spreads, like 20-30 fps or more, and poor groups on target to match. Seemed good for a while, but it didn't last. And my first shot after a half-hour rest will be over 30 fps slower than the next 10. With some fiddling I've gotten a smaller ES, like 7, for 8 or 9 shots, then the next is much faster. So shots 1-8 average 935, then shot 9 or 10 will be 960. Seems that little by little as I adjust hammer or valve, I delay the high shot until later in the string. Used to be the 5th, then 8th, now it's 10th.
 
Hi Khornet,

I'm in the middle of tuning for slugs so FWIW, I can share my notes...

Same rig as you (yup, it's black :) ) except that I am currently using the pellet barrel and will change to the "slug A" barrel after a bit of experimenting. 

I've tuned to the 25 grain JSB Monster Redesign pellets and will be switching to the JSB KnockOuts (they are the same weight). I'm hoping to find a tune that will let me switch between pellets and slugs. 

Agreed, reg then hammer spring , then valve timing. Note that the reg pressure gages are often inaccurate so bar numbers are for reference only. Likewise, the numbers on the HS dial are relative to how the Allen screw on YOUR gun is set (and the strength of your spring) and the lines on the valve are vague at best. I take measurements and go by those dimensions. 

Was tuned to shoot JSB 18.13 grain pellets @ 939 fps (reg 75 bar, HS .742"; Valve .323") ES under 7 & SD under 3 (typically).

I (think :) ) I'm looking for 930 to 950 fps with slugs. I started tuning for JSB Monster Redesign pellets, I increased my reg pressure in 10 bar increments until I reached my starting velocity of 970 fps at MAX HS (.575") . Then tested (recording FPS, ES & SD) at MIN thru to MAX looking for stability (low SD) and a velocity of 955-960 fps. Found that with a HS setting of 5 (.592") then I tuned the valve down to 3 1/8 lines (.337") to my current tune of 947 fps. Typical ES and SD over 25 shot strings is around 7 and 2.3 respectively. Groups are good with pellets and now that I have the weight (reasonably) dialed in I'm ready to start looking at slugs.

Just as a thought, I have often wondered if the sudden changes in slug velocity (and accuracy) could be attributed to them not feeding consistently - that they are loaded canted and not seated to a consistent depth. After all, pellets are supported/guided at the front and back. Slugs, by their shape are only aligned by the back - that might be a problem, especially on the shorter light weight ones. The changing tension in the magazine spring could affect feeding as well. Just saying. 

Done rambling. Hope this helps. 

Hank
 
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Yes, I've watched those videos. More than once. Yes, I know about listening to the gun, and have observed the phenomenon many times while tuning for various projectiles and purposes. I've read all of Bob Sterne's technical articles. I'm not asking how to tune an air rifle, but about a specific tune I've heard mentioned here a few times, using a higher reg pressure to get the same velocity. I think someone reported on this forum, for example, that he was even using a tune giving 8 foot pounds with regulator at 150 bar. How does that happen?

I'm not looking for high power; 44 ft lb is plenty for my needs. This high regulator business is being recommended as a way to get better accuracy, which is what I'm after.
 
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Figured by your comments that you are well researched and know what you are doing. Shared my notes as another point of reference for what worked for me and how I got there. :)

Roelf Vorster (Air Tac) was talking about the benefits of using a high pressure tune (more energy stored per volume). My guess is that it applies more to the people who are shooting heavy slugs at high speeds (34 g @1000 fps IIRC). Would think that it would make for interesting harmonics. 

Don't know if you saw BobO's video on tuning for .22 caliber slugs and it seems that very fine adjustments are required to find the sweet spot.

" 8 foot pounds with regulator at 150 bar" - sounds like a science experiment to see how far one could push things, can't see that as being practical.

Like you, I don't need a lot of power - my favorite rifles are low power 10 meter rifles (FWB 300 & FWB 603 and a TX200 tuned to 12 fpe), they are just so precise - even out to 25 yards.

My current tuning is more for curiosity and experience than anything else. I have a couple of tins of slugs and once they are gone I will be going back to my 35 fpe tune with 18.13 pellets. On the rural property where I live I can grab my .22 rimfire or my 30-06 if I want power.

I like pellet guns BECAUSE of their low power :)

Cheers and good luck with your tuning!

Hank
 
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For better consistency/accuracy very high reg tune isn’t actually the best approach. Which tune to use is integral part of how to tune which needs good understanding of each components ans their effects on each other and the end result. Depends on the tune/end result you want the how to tune part will change so it’s not that simple.


Yes, my crown was set at 150 bars and shooting 8 FPE. Why? Because I want to shoot 8fpe or 650FPS with 7.9f pellet. Sadly the low TP setting limits me to about 600 FPS regardless how much reg and HS so only way to detune to the power level I want is to raise Reg to max and lower HS to Min and that got me to where I wanted. The HS is so weak it can barely open the valve. I’m since changed my tune to 130 so with TP at medium 7.9g flies at 800fps and with TO on low it’s right under 600FPS but very consistent. This isn’t a tune for everyone while it’s extremely efficient it’s certainly won’t be most consistent and reg creep shows up VERY easily. This is a bit of extreme case because of the changed FX made to the crown.


For max accuracy/consistency you need less reg and tune your gun to the sweet spot for that reg pressure. If it’s a bit low or high then adjust reg and HS so the sweet spot is slightly higher than your desired speed then crank down the valve till at your desired speed. This process is well documented in those 2 videos and this approach will give you the best consistency and accuracy. However the barre length also has direct affect on the sweet spot.


The beauty of high reg tune is the versatility, on my Dreamline with 500mm barrel the low is at 600FPS and with a turn on 2 knobs I can shoot heavier 12.5 slugs at 900fps and the Hs isn’t even maxed out. 
 
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