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Tuning Impact M3 + Slug Power Kit - Understanding Tuning

I recently purchased an Impact M3 700mm/.25 and have been having fun shooting it a ton over the last week with the JSB Exact Kings that Utah Airguns tuned it for. Today, my Fx Pocket Chronograph finally arrived, so I was excited to finally get it tuned for the JSB King Heavies and eventually the NSA slugs that should arrive soon. In preparation for all of this, I installed the Slug Power Kit last night and that is where the confusion started...

As you may know, there is a lot of discussion about how the Slug Power Kit includes all sort of stuff that may or may not be needed for the M3, depending on who you ask. So, I started by installing everything except the pin probe (no slugs yet). Like many others before me, I quickly realized that this resulted in horrible performance. The valve spring and ball took more than 100fps off my shots and it wasn't super tunable either.... yuck. So, I pulled those out and put the stock spring back in and things were looking better until I started to try and tune it...

My first attempt was to bump the 2nd reg up to about 115bar from the 90bar it came at, then I backed off the valve to about 4.5-5 and dialed up the hammer spring. After shooting a few rounds, I was already hitting 960fps with the Heavies. So, I dialed the valve back a bit and settled in at 875fps and thinking that this whole tuning thing was pretty easy. Then I dropped my macro wheel from 16 to 1 to see what sort of adjustability I had and I was shooting at about 865fps. Something had to be wrong.

I spent hours trying different hammer spring adjustments, dialing the 2nd reg back a bit more and adjusting the valve. What I found was that the valve had a big impact on my fps, but the hammer spring made nearly zero difference. And then I had my moment of clarity.

Essentially, all I did with the Slug Power Kit was add some weight to my hammer weight and compression to my hammer spring, and when I started thinking about what this does and why it is necessary, I finally started to understand that this is essential to aid in forcing the valve open at higher pressures of the regulator. I had already read about this feature of the kit, and I have seen some users pushing the 2nd reg up past 175bar to really push large slugs, but what I wasn't thinking about was what the effect of this would be at lower pressure. Well, all that weight and added tension will just blow right through the valve tension at lower pressure. In fact, it seems to do this so effectively that it almost doesn't matter what you set the hammer spring tension at! You essentially get the same results regardless and are left with only the valve spring adjustment.

To test my new found understanding, I pumped by secondary regulator up to 150bar, which seems like way more than I need to push King Heavy pellets, and I also pushed my 1st reg up near 190bar to keep some delta between the regs. After dialing my valve and hammer spring back to reasonable levels, I started firing off some pellets. As expected, I was pushing the pellets at nearly super-sonic at 16 on the macro wheel, but I was interested to see what would happen when I dial it back to 1. After shooting 5 shots at 1 on the macro wheel, I was averaging right around 680fps... success!!!

Anyway, I'm sure this is easy stuff for some of you, but I learned a lot about my airgun today. Also, reading through tons of posts on this forum, and watching lots of tutorials, has been extremely valuable. So, thanks everyone! Couldn't have done it without you.
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May 6, 2020
CT, United States
    I think you’ve got it! Its a journey a lot of us have gone through. I had a similar experience to yours with the Slug Power Kit on a Wildcat MK3. Luckily I had already learned some fo the lessons you outline above. For me it was a Dreamline with WAAY too much hammer spring tension that led me down the whole journey. My velocity would increase when I lowered the Power Wheel and was higher at position one than at position 5. People here were super helpful and I learned an immense amount about how these valves/hammers/springs/pressures etc. work. It made life much easier when I started tuning my Impact.

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    I can't thank you enough for sharing your experience with tuning your M3. I replaced my standard plenum with Power Plenum 720 and enlarged the port holes on my barrel and pellet probe (simulating the same performance achieved by the slug kit). And then I tried turning it and realized I lost at least 100 FPS of power...I also experienced the same problem with the hammer spring tension, from 1-16 no difference. After 8 hours of fiddling and a ton of ammo wasted I gave up. My "moment of clarity" never came but after reading your post I have hopes that I can fix it. 

    So you left your second reg at the final press of 150 bar? What is the noise level at 150 compared to 90 bar? I'm led to believe the noise will increase linearly with the pressure increase.

    I guess the whole philosophy of big plenum, low-pressure, low noise, and increased shots count doesn't really apply to slugs, which need high power...

    Yes, I ended up leaving the 2nd reg pressure at 150 bar. The noise level, especially without a moderator, went up substantially. With the macro wheel at 16 it sounds like a powder-burner. However, I have been shooting JSB King Heavies (33.95gr) at Macro 8 and A&A Fields (25.4gr) at Macro 3 with great results. This gives me about 870-885fps in each and the sound level is not bad at all. Maybe still a bit higher than at 90 bar, but it's pretty quiet with the DonnyFL in place.

    Basically, what I have found is that hammer spring tension and regulator pressure need to increase in tandem. The heavier your hammer assembly, the higher your baseline pressure will need to be in order to oppose the hammer tension. Also, increasing the barrel port will also likely require higher pressure to get more air volume. The smaller port will act like a jet that focuses your air volume, so you will get more pressure with less volume. The larger port allows you to push more volume, thus producing higher velocities for heavier loads, but it's also going to require more air and thus more pressure in the plenum.

    So, I think you should crank up the 2nd reg pressure and see what happens. I bet you'll start to see much better results. Good luck and let us know how it goes!
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