Tuning Tuning an FX impact 2 for accuracy while maintaining high pellet energy.

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Oldandintheway

Member
Sep 26, 2021
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VA, United States
Gentlemen; I own an airforce 50 cal Texan LSS and it will put five 250 gr bullets in the same hole at 65 yards. I also own an FX Impact 2 that I am getting really frustrated with. It won't come anywhere near the Texan accuracy. I have it tuned so that it will repeat the velocity of 860 FPS and each shot within 5 FPS, when I use carefully weighed pellets. Plenum pressure set at 135 Bar. Rotating ring pressure set at 5 (max). Hammer pin set at just barely touching. It would get about 3 inch groups at 100 yards. Not good.

I moved the hammer spring restrictor in about two full turns. That reduced the pellet velocity to 715 FPS, with a deviation of only 7 FPS., and dramatically increased accuracy. That, however reduces the total energy by a lot (down to about 90 F LB). I'm happy with the accuracy but not with the total energy.

My question for this esteemed group is, Does the velocity of the pellet, regardless of what combination of power from the two power setting possibilities and the location of the hammer spring limiter determine the accuracy of your rifle, or does the exact combination of those three settings derive a good accuracy? I know that that's a little hard to understand, so let me try another way to ask this question. Lets say that you had your plenum pressure set at 135 pounds, and your rotating ring power setting at 3 and your hammer spring limiter set at 4, arriving at a pellet velocity of 850 FPS. You then changed your plenum setting to 160, your power wheel setting to 1 and your hammer spring setting at 1, and you still got a pellet velocity of 860 FPS. (The settings numbers may not be logical, but they are being used for discussion purposes only.) Is it possible that one combination would get a good grouping and the other setting combination would not get a good grouping, even though they both were sending the pellet at the same 850 FPS? Like all airgunners I would like to get as much pellet velocity, and energy (knockdown), , with as little noise as possible.


 
For the most part if your pellet comes out at the same speed it will group near the same. I think it's a harmonic balance thing.

Lighter to medium weight pellets often seem to prefer the slower speeds in the 830-860fps range, for best accuracy, and the heavier pellets can go up as far as 890-900+fps and still be accurate. Some of the heavy ones actually like to be shot fast or they won't group.

Shooting the appropriate JSBs for your caliber, barrel clean and barrel probe o-ring is in good shape?

Then again I shot my .25 700 slug A liner at 740fps and got sub MOA at 100 yards from the 33.95 JSB pellets and NSA 33.5, 26, 26,2 and VK 34gr slugs at that velocity, and the same tune,as well. They all did equally as well near the 830fps mark and nearly as well at 860. The slugs grouped the same at 960fps.

Impact and not Dreamline but the barrel is the same and that is what counts.


 
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There are so many parts of the accuracy equation but the speed that Diabolo pellets shoot is one of the major ones. Also the speed it goes through your gun makes a difference in harmonics as well. Both will affect accuracy drastically.

This video will show how the difference speed affects things at close distance. 

https://www.youtube.com/watch?v=KeaUeB7urAo&ab_channel=AimSmall

Further distances will really show you how well you are tuned as pellets shot to fast or to slow will spiral out of control.

https://www.youtube.com/watch?v=QFou_4VqLBY&ab_channel=TedsHoldOver


 
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https://hardairmagazine.com/ham-columns/the-definitive-index-to-bob-sternes-ham-technical-articles/

I hope this helps you. There’s a couple of really useful articles inside of this link that helped me understand tuning especially for regulated PCP. Using the hammer to tune 3 to 5% below max velocity of your regulator pressure (as long as your hammer is not experiencing valve lock) is one of the most useful bits that I gathered. This helps minimize turbulence behind the pellet as it exits the barrel. Thus maximizing efficiency and accuracy. 
 
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Really good question, and I can't answer it, but my gut instinct says that you need to balance the hammer vs reg pressure at the speed you want for the pellet/slug you are using.

This means you have the optimum, most efficient amount of air burst behind each shot. Of course if whatever way you achieve the speed gives you the accuracy then bingo, but I personally would go down the course of ballancing those two factors first. I certainly don't claim to have mastered it, but that's the theory I am following.



Paul.
 
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Each pellet or slug will have a preferred velocity from a specific barrel, and how it is achieved usually doesn't matter, at least from an accuracy perspective. I believe the best approach is to first find that velocity by simply playing with hammer settings. If you are then at an inefficient tune, or one that doesn't have a desirable ES, then tune however you want to that identified velocity, adjusting the reg set point as necessary. And you can't make every pellet or slug shoot well from every barrel. The pellet/barrel combination is more important than all the tuning you can do. 
 
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