Dry fire "tuning"

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tor47

Member
May 18, 2018
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Other, Norway
 I wanted to say something about regpressure and hammerstrike in comparison to harmonics, and my litle experiance with it. I am not an experianced tuner, but I am a owner of both a fx crown in .22, and a wildcat mk2 in .177. The wildcat has a huma reg in. On both guns I have tried velocity ranges from 800, and up to about 900 fps. I mostly shoot at 25m, I have spent allot of time with the wildcat, where the goal is to get as close to hitting or touching the same hole on a magasine of 8 shot. What I noticed from first tuning at the "knee" was that even the extreme spread was very good, the accuracy was not quiet what I hoped for. As I adjusted regpressure up about 5 bars at a time, I covered the same velocity range as before, by turning the hammer spring screw, and also went up to the new plateu. I have btw drilled hole in my stock, so I can adjust without taking stock off. 

But the point is: At first the gun did shoot best at about 820-830 fps, even I could go much higher. Also the extreme spread was not that bad, even I was much lower than the "knee" velocity. Somewhere inbetween the "good range" and the plateu, the ES actually was worse, and also the accuracy was worse. I also discovered when I took a dryfire shot, the crosshair in my 20 magnified scope hardly jumped at all. Found it interesting that the gun actually "recoil" with no pellet in it. So when I upped the regpressure 5 bar more, the same thing happended again, the gun jumped less when dryfire, but with hammer striking harder as before, and the gun shooting faster. But only within a limited range. when I went down to the previous 820 velocity range the ES, and accuracy was not as good as before. Also the crosshair seemed to move a lilte more than before when dryfire. So in the end I did not care for the plateu velocity, or tuning for the "knee" anymore, and simply went for the velocity range where basically the crosshair had lilte dryfire movement, and tested more or less around that. So as increased pressure, and adjusted for the least dryfire movement range, both accuracy and ES seemed to drop in place by itself . Now I have for the moment setteled on 115 bar, shooting 10 grain JSB at 870-880 fps. Think I will keep it there for the moment, as shot count is good, and accuracy is also close to the one hole in 8 shots I aimed for, on a none windy day.


 
The crown has been a lilte more challenging, as the barrel fouled up very quickly, and could not shoot long sessions before having to clean. So I have started to lube the pellets with silicone oil, and it seems to work better.

But so far the same patern seems to repeat itself from when I tuned the wildcat. I started with a repressure of 110 bar shooting 18 grain JSB with a max speed at around 860. Started with velocity at about 820, as my current hammer wheel setting on min did not allow for lower. As I increased pressure 5 bars, the gun did shoot better on the lowest setting, and accuraccy got worse on higher. Now I am at 120 bar and the gun seems to shoot best at around 830-840 fps. Max speed on that setting is around 890, but it doeas not shoot as good if I get closer to the "knee". Crosshair has hardly no movement when dryfire at 830fps. I am planning to shoot faster, so my next step will be to go 5 bar higher, and if all hold true as before the next accurate velocity will again be around where the crosshair moves less when dryfire. At least I hope, so I do not make a fool of myself here in the forums:)

I will add that if you stay much below plateu, or the "knee", the gun might not be as accurate on the first 2 shots, if pressure in reg chamber has incresaed over night. So it will probably require 1, or 2 dryfire shots first.

I will also say that I am not critisising all I have learned from reading the forums about adjusting reg and hammer for "knee" veocity. It is probably very usefull when you plan to shoot below regpressure (which I do not do on my setup), and probably many other benefits also. So I still recommend trying that method also.

I just wanted to share the "dryfire" prinsiple which I for the moment think can be usefull on maybe FX guns. Would be interesting if someone else took the time to take a dryfire shot when you test your settings, and share if you find the same patern. You might not notice it if you use a low magnification scope.
 
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I have btw drilled hole in my stock, so I can adjust without taking stock off.
Click to expand...

Tor, 

Nice little mod there. Can you post a picture of that? Curious as to enter you drilled a vertical hole or one at an angle. Im always annoyed I have to remove the stock to fine tune the Crown. It’s a real pain in the ass compared to an Impact with its Valve adjuster. 

George
 
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Hi Tor, thanks for sharing your observations. The way you have articulated your choices tells me you have a solid understanding of tuning so I'd like to offer some thoughts on this approach.

By tuning somewhere down the slope of the velocity curve, it's possible to achive a very pleasant shot cycle indeed. Low recoil, low noise, high shot count...all of which can be credited to very short valve dwell. In some cases it may also eliminate air-wasting hammer bounce which will further improve those 3 qualities.

I like it a lot for backyard shooting at modest ranges. It can work very well for paper targets and tiny reactive targets (e.g. aspirins or Mardi Gras beads out to 30 yards or so), and for super-stealthy pest control. At this state of tune, the gun is operating at partial valve lock. In other words the valve is barely being knocked open. If the hammer spring tension were continued to be backed down, the valve would open less and less, the velocity fall more and more, and become ever more quiet...until eventually it would not fire at all.

As you pointed out, the downside is that the extreme spread grows. Minor variations in how the hammer strikes the valve (friction, subtle sear drag, etc.) get amplified. But here's the thing, if you don't move too far down the slope and keep the shooting distance modest, groups may not be affected in any noticeable way and the reward is a very enjoyable shooting experience because it's so quiet and free of noise and vibration.

To relate this behavior to something perhaps more people are familiar with, it is the same thing that is happening at the beginning portion of the bell curve of an unregulated rifle. Especially if the gun is overfilled past its sweet spot, it starts out slow and quiet. It uses very little air. If you graph the shot string, the first part of the curve will usually have noticeably more "jaggies" than the other side of the bell curve. For example:



To give a practical example of how useful this type of tune can be, some years ago a friend asked me to come over to his home in the suburbs to cull an overpopulation of gray squirrels. He said the longest shot would be about 30 yards. I had just finished setting up a .22 cal rifle at 30fpe that I was anxious to press into service but that power level was way overkill and I wanted to be discreet on account of the neighbors. So in preparation the night before, I set up the cronograph and dialed back the hammer spring an arbritrary amount. The first trigger pull greeted me with a metallic "tink" of the hammer striking the valve stem and a comically weak sneeze of a report. I looked down at the chronograph. About 700fps. Huh? No way. That can't be right. It sounded so weak I figured the pellet must've dribbled out the end of the barrel. It was only after putting a few pellets into the backstop of my 25 yard target that I was able to believe the chronograph.

I shot a few groups and noticed the velocity was jumping around but the results were more than adequate for the task at hand. The next morning the first squirrel fell to a brain shot at 22 yards. His buddy 4 feet away just looked quizzically at him flopping around on the ground and went back to foraging. I handed the rifle to my son and he took the second one with just a few seconds later. That morning the new rifle went 6 for 6 and drew no unwanted attention from the neighbors.

I don't remember how bad the ES was but that was a pretty extreme example. The energy was cut in half from 30fpe to 15fpe. A more modest reduction would be better for a broader spectrum of shooting duties.

Lastly, to your point about experimenting with various regulator setpoints and HST settings, that may well reveal a different excitation of barrel harmonics that erases a portion of the velocity spread. Some pellets will leave the barrel faster, some will leave slower. Let's say the faster pellets happen to reach the muzzle when it's near a node (zero crossing). Meanwhile if the slower pellets happen to reach the muzzle near the peak of an antinode, their POI may match more closely than the extreme spread would predict. At least at modest distances. At greater distances it would show up more.

For those like me who do a good portion of their recreational shooting inside of 30 yards or so, it's definitely something I encourage you to experiment with. For guys with an indoor range, even more so.
 
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Thx for the explenation. I found the extreme spread can be pretty good, within a limited hammer adjustement range. And that is usually where the cross hair seems to jump less when dryfire also. In some cases the es get wider as I work the hammer force up closer to the plateu, but then it tighten up even closer the more I adjust. But for my use the end result seems to be better with a litle wider es, but staying lower with the hammer force.
 
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When I think about what you said about partial valve lock, it think it makes more sense to me. As in my example with the wildcat the gun was "recoil less" at 820-830fps. When I then tried with higher speed turning in the hammer adjuster, the hammer strikes harder, but at the same time makes bigger movements inside the gun causing more vibration. In my case thouse increased vibrations impacted the gun in a negative way leading to less accuracy. So when I increased regpressure and turned in hammer spring again, the hammer was moving inside the gun the same amount of distance as before, but it shoot faster, because of higher pressure instead. So the end result seems to be it shoots with less vibrations at higher speed, than it would do with the same speed with a lower pressure and wider hammer movement?
 
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The reason I noticed the gun was more stable when dryfire also, was that the wildcat has no way of telling you how many rounds are left in the mag. So I sometimes did aim down the scope on target, and took another shot. After that I startet to intentionally do dryfire shot after adjusting, to see if the gun seemed stable or not. When I at the same time did shoot on paper, trough the chrono, it just seems to me both accuraccy, and ES are good in that range. 
 
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