FX I can't tune this… Impact M4 extended .22… with 18.13 grain pellets…

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Doc_pafumo

Doc_pafumo

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Oct 10, 2024
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Badajoz, Spain
I might know the answer to the problem. I bought the rifle a year ago, and those who understood already warned me about its power. The beast is the Impact M4 extended (800 mm) with a superior heavy barrel. I know it's designed for slugs, but I don't know what it is about the JSB 18.13 grain pellets that everyone wants to use them in every rifle. When I was warned, "it’s for slugs," I trusted I could find a valid tune for the JSB 18.13 grain pellets at 870 fps. I’ve watched videos, and today, with more hope than faith, I started the tuning process. I began by lowering the regulator to 60 bars, with the QST at the minimum and the front valve at the 4th line... the speed obtained was over 920 fps... I increased the QST and, as I feared, the speed didn’t increase, it even dropped... I suppose it’s too much hammer for such low regulator pressure. I raised the regulator pressure by 5 bars each time, which increased the speed, but with the same behavior when raising the QST. When I reached 110 bars, the speed increased as I raised the QST, but the speed was above 1000 fps. My question is, is there any possible way to tune this rifle with the JSB 18 grain pellets to 870-900 fps? If so, should I change the hammer or spring, the barrel to a 600 mm… or maybe lower the front valve at the beginning of the tuning? Impact M4 uses tungten Hammer?? Or, if it’s not feasible to use the JSB 18.13 grain pellets at those speeds, should I give up and go with slugs? Maybe I made a mistake choosing this Impact model…
 
Doc_pafumo said:
I might know the answer to the problem. I bought the rifle a year ago, and those who understood already warned me about its power. The beast is the Impact M4 extended (800 mm) with a superior heavy barrel. I know it's designed for slugs, but I don't know what it is about the JSB 18.13 grain pellets that everyone wants to use them in every rifle. When I was warned, "it’s for slugs," I trusted I could find a valid tune for the JSB 18.13 grain pellets at 870 fps. I’ve watched videos, and today, with more hope than faith, I started the tuning process. I began by lowering the regulator to 60 bars, with the QST at the minimum and the front valve at the 4th line... the speed obtained was over 920 fps... I increased the QST and, as I feared, the speed didn’t increase, it even dropped... I suppose it’s too much hammer for such low regulator pressure. I raised the regulator pressure by 5 bars each time, which increased the speed, but with the same behavior when raising the QST. When I reached 110 bars, the speed increased as I raised the QST, but the speed was above 1000 fps. My question is, is there any possible way to tune this rifle with the JSB 18 grain pellets to 870-900 fps? If so, should I change the hammer or spring, the barrel to a 600 mm… or maybe lower the front valve at the beginning of the tuning? Impact M4 uses tungten Hammer?? Or, if it’s not feasible to use the JSB 18.13 grain pellets at those speeds, should I give up and go with slugs? Maybe I made a mistake choosing this Impact model…
Click to expand...
Bring the reg to 70b, micro around 2.2 and valve line around 2.5 and see where that lands you, from there work your way up or down.
 
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I know for sure that different variations of FX airgun models are configured differently. Different hammers, springs, etc. Your M4 sounds a lot like the Wildcat I have and aside from mine having two regs, they're really similar. My hammer is stock at 11.4gr and the 52mm spring, so, as far as I can find, those are the lightest factory parts. I tried to shoot 18gr pellets, and I could get close, but the gun really didn't like them. I experienced the exact tuning behavior, and I plan to go back and try again, but my journey indicated trying 25gr MRD's and the tuning went just as you would expect, without the funky ups and downs of reg changes causing velocity anomolies. I simply went up to around 900 fps, and the es settled right in to around 10 fps, easy peezy.

The long and short is, I "believe" it can be tuned lighter, but will take some work and trial and error. But get into it's sweet spot, and you're golden. At least on the WC, FX states on their website, that if you want to shoot "lighter" pellets, you might be happier with the standard WC as opposed to the bottle version.
 
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The 18.13 gr JSBs are great in 30ish fpe PCPs. Your .22/800mm MK4 is a slug gun and likely configured for 30 to 40 grain slugs and 60 plus fpe.

I see 3 options...

1) The simplest/easiest is to try heavier pellets, .22 pellets are available in 25 to (IIRC) 28 grain weights.

2) Change the configuration of the MK4 to suit lighter projectiles.
- lighter hammer (and spring?)
- rotate the transfer port from the dual port slug position to the single port pellet position.

3) Or just shoot slugs

Good luck!
 
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Doc_pafumo said:
Sorta problem 1. The beast is the Impact M4 extended (800 mm) with a superior heavy barrel. I know it's designed for slugs, but I don't know what it is about the JSB 18.13 grain pellets that everyone wants to use them in every rifle.

Solution. When I was warned, "it’s for slugs,"

Problem 2.My question is, is there any possible way to tune this rifle with the JSB 18 grain pellets to 870-900 fps?

Solution. change the hammer or spring, the barrel to a 600 (700) mm… Or, if it’s not feasible to use the JSB 18.13 grain pellets at those speeds, should I give up and go with slugs.
Click to expand...

My though it try the 25 grain pellets and hope for the best, or go slugs. If youre hell bent on pellets, you gotta change parts.

  1. Remove bottle and degas gun.
  2. Move Macro to middle position (8) and Micro to (3).
  3. Move Valve Adjuster to right over 4th line.
  4. Set First Reg to 150 Bar. You have no first reg
  5. Set Second Reg to 50 bar as starting point.
  6. From here, slowly shoot and adjust your second reg a 1/8 to 1/4 turn at a time until you reach about 10 to 20 fps or so below desired FPS.
  7. Now from here use your Macro wheel to hit desired Fps within 5 to 10 FPS.
  8. Once you are in that 5 to 10 FPS range, use Micro Wheel to make with in 5 FPS or so more consistently.
  9. Now, once at FPS needed, SLOWLY turn Valve Adjuster IN (1/8th or LESS) to further add to consistency. YOU SHOULD NOT HAVE TO TURN IN MORE THAN HALF OR SO.
 
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corny said:
the plenum on that gun is simply to big for that low power tune.
The valve will become slow and sluggish under those low pressures. Accuracy will be affected.

I’ve heard of ppl partially filling the plenum. So that can up the reg pressures for better valve performance.
Click to expand...
How is it possible to partially fill the plenum?
 
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I mainly shoot 18gr pellets out of my Taipan Vet2 and RAW and both are shooting them @ 975fps very accurately. If your accuracy is great @ 920fps why not keep it @ that speed? JSB 18gr pellets are accurate @ many different speeds and are very versatile because of that. Like said above your gun is built for heavy projectiles.
 
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Couple things.
Going with the longer barrel and high power rifle makes it harder to shoot lighter pellets slowly. If someone set it up specifically for slugs, then there is a chance they went with the pin probe and heavy hammer as well, which would make your job even harder.

Something you could do to help choke the air down and get your speed down is the remove the barrel and twist it 180* to be sure the air is coming through the pellet air hole instead of the slug hole. The pellet side is just a round circle and the slug side is the larger square with the split in the middle. If your has this, it can help you slow things down.

I would prefer to have a shorter barrel 500mm or 600mm at most if I was shooting 18gr. To get low enough to shoot 18gr, it will be hard to have a flexible reg setting to also shoot 25gr or more with just hammer adjustments with longer barrels.
 
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Doc_pafumo said:
Yes, i bought it in "extended" version. I think hammer is too strong...Otherwise, although the barrel is "superior heavy" the accuracy is good with pellets
Click to expand...
You are probably spot on. Guessing with the 800 barrel it is probably set up from the factory geared toward slugs. Probably need a lighter hammer.
 
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While waiting for the 600 mm non-Heavy barrel to arrive, I tried shooting with 25 grain pellets at 950–960 fps. I raised the regulator to 105 bar and gradually set the hammer spring to 2. After reaching a plateau velocity of 990–1000 fps, I lowered the hammer until 960 and closed the VA until it started to affect velocity… around the 3rd line. The spread is now 12 with an SD of 2.5 and good accuracy. I like it.
 
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Doc_pafumo said:
How is it possible to partially fill the plenum?
Click to expand...
Little difficult as u would have to tear down the gun….but i would do the calculations and drop a piece of well fitted nylon into it. Don’t want it rattling about in there
In this case I’d shoot for around 30cc of plenum. But that’s another just me :)
 
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