GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: Tweeter on September 26, 2017, 02:38:03 PM
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WHen my wildcat falls off the reg around 140bar the fps starts to climb. Does this mean the reg is set too high? May yank it out and lower it but not sure how much to turn the adjustment nut.
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Yes, it's too high. If the velocity drops right off the reg, then it's low. If the velocity spikes, it's high. It should have a gentle rise and then taper off. Sorta like a mini bell curve. You can tune the reg to make it rise as much as you want. More rise, more shots, just like a bell curve.
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Thanks Chris, I've been reading so much reg info lately I think my brain is scrambled haha. Some of this stuff is overwhelming, especially when all you want to do is go shoot!
I'm thinking by lowering the pressure and getting it set right the ES should shrink too? Not terrible right now at 21fps with my last 48 shot string but could be better?
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If you are tuned just below or at the knee of the curve, you should see a slight rise in speed as you fall off of the reg. Putting you at or near optimum efficiency.
I would highly recommend reading this thread - https://www.gatewaytoairguns.org/GTA/index.php?topic=74919. (https://www.gatewaytoairguns.org/GTA/index.php?topic=74919.)
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Here's another question. As it is now I get an average of 880fps with 18.1 jsb's and when it falls off and I get to around 125bar the fps is around 900. So if I lower the reg to 125bar it will allow me to lower my HS tension to get back down to 875-880?
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You are getting 900 feet per second at 125 bar. But that is unregulated and the whole tube is being used. So if you regulate to 125 bar, you are reducing the used volume to the plenum volume. So you will likely lose some velocity.
Keep in mind, hammer force, controlled by the hammer spring tension, can do similar things to a shot string. If the hammer hits too hard, the string will drop right off the reg, too little and it will show that rise and fall. It's a fiddly game of balancing regulator setpoint and hammer spring tension. If you like the velocity you're getting out of the gun right now, I would try lowering the regulator setpoint about 100 psi. Then shoot a string.
If the velocity drops, but you still get a spike, then increase hammer spring tension and it may bring the velocity back up to where you like it and get rid of the spike.
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I just shot a string from 170bar down to 110bar and Ill post the results in a bit. The ES isnt horrible I guess but what I dont like is the SD. An example: even though my ES is around 20fps I might get 870 one shot and 890 the next. I want my mutant back... :-[
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I agree with what has been said thus far, and some of what I'm about to say is repetitive just to make the concept flow better. The increase in velocity as the pressure falls below the regulator's setpoint arises from having too little hammer strike. In overly simplistic terms, the hammer is not fully knocking the valve open. Once the pressure falls low enough, it knocks the valve open with more authority--more lift and dwell--which results in a higher velocity.
The usual recommendation is to choose between two paths. The first is to tune it for higher velocity and lower shot count. The second is to make adjustments to keep the current velocity but eliminate the velocity spike that occurs as pressure falls below the setpoint.
Path 1 is very simple and straightforward. Just keep adding more hammer spring tension until you find the peak velocity. (Depending on the design, that can be as simple as turning a screw to add spring preload. In others, it may mean adding shims to the spring or stepping up to a heavier spring) Then back off until you are at about 97% of that peak. So if the max was 950fps, back off until you're at about 950*0.97 = 922fps. With that done, the velocity will remain stable a bit below the regulator's setpoint and then begin a well-controlled rolloff as the pressure continues to fall.
Path 2...the usual recommendation is to reduce the regulator setpoint, then adjust the hammer spring tension as described in #1. Keep at it until you find the regulator setpoint that gives you the desired velocity.
However, and this is the real point of this post, there's another way of doing it that does not require adjusting the regulator. You do it by restricting the transfer port. How? Again, adjust the hammer spring as described in #1. That satisfies the condition of fully knocking open the valve in the interest of providing a well-behaved velocity rolloff below the setpoint, but the velocity is higher than you want. So now restrict the transfer port until the velocity comes down to where you want it. I don't recall if the Wildcat has an adjuster for the transfer port but if it does, that makes it super simple.
Granted this wouldn't be the preferred way to take a 1000fps fireball down to 500fps for doing 10 meter practice in the basement but for modest changes it works great.
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Thanks Jason for putting it in simple terms that I can understand lol. I only seem to be getting a slight increase in speed when going off the reg so just add a little hammer tension until my speed stays consistent even when off the reg ?
I think it would help if I post the wonky shot string I shot this afternoon so you guys can see my dilema. Maybe its not as terrible as I think.
In the middle of the maine woods now taking a break on my dirtbike but I couldnt help but to check in haha.
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No problem, sounds like you are getting close.
Regarding the SD, it can be from a lot of things but the things that affect ES are usually interconnected.
First and foremost, make sure your hammer spring tension is adjusted as described above...near peak velocity.
Beyond that, it's fine attention on the hammer and anything that influences it...smooth hammer, smooth tube, lubricated appropriately, and a trigger that breaks clean with no sear drag (i.e. not too aggressive on the overtravel stop).
But with that said, let's put it into perspective. A 20fps ES represents only a bit more than 2% for a muzzle velocity of 900fps. That's about as good as can be expected from the pellets themselves. So before laying it at the feet of the rifle, it may be useful to take 20 pellets and measure them to see if there's a correlation. Head sizes to 0.0005" and weights to 0.1gr should be sufficient resolution. Absolute accuracy isn't as important as relative numbers. Lighter pellets with smaller heads will leave the muzzle faster than heavier pellets with large heads.
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Yes to be fair to the gun I have been using up pellets that dont seem to be its absolute favorite (jsb 18.1 w/ thin skirt) because thats all I have until my pellet order gets here. I have read from a couple other wildcat owners that as soon as they switched from the 18.1 to 15.9 it improved their ES quite a bit. Maybe mine will follow suit.
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Here is the string I shot today from 175bar down to 110bar. pretty sure the gauge on the gun is accurate as it matches my hill pump and the hill pump matched my s510 gauge.
175bar with jsb 18.1
840 the usual low first shot i think from shooting it below the reg set point before refilling
873
878
876
868
866
864
879_____165bar
873
887
875
892
890
878
875
888_____155bar
882
883
870
873? I wrote 813 but that wasn't it
879
883
887
882_____145bar
895
890
904
894
896
899
899
901_____132bar
894
906
903
894
895
888
891
886_____120bar
877
868
859
849
828
828
814
814
There it is, I apologize I don't know how to make a graph so it is easier to read. It actually seems to get more consistent for about 15 shots when it comes off reg around 140bar (where it is supposed to be set from the factory). Actually FX has since started regulating the wildcats at 125bar but mine is from before the switch.
Edit: The ES for this string is obviously higher than normal because the whole off the reg business. ;) Just wanted to point that out. Usually I fill to 220bar and shoot down to roughly 150.
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The hammer spring is very easy to adjust on this gun. Just an allen key to the rear of the action. I could adjust the transfer port size by either spinning the barrel to partially block it off or moving the probe forward or backward some which would interrupt the flow of air. It would be a tedious job and hard to replicate every time I had to remove the barrel or loosen the probe set screws so that's out. Adjusting the reg is pretty straight forward as well but without a reg tester I'm not sure I'd have the patience. Actually I KNOW I wouldn't have the patience.
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That's hard to say honestly. It doesn't really go that much higher. The first about 10bar is what's hurting you honestly. A few 860s then after that seems to settle into 875-890 and then raise to 900 after it falls off the reg.
I do what Jason does as well for the most part. I tune regulated guns to the pressure. He uses 97% of the max speed and I just go 20fps slower. Not much difference if you do the math but get the point. When I just tuned my .20 Mrod I got lucky the first time (never happens!!) And my Max speed was 880fps at my set point and I like to tune my guns to around 860fps normally so it was perfect. I start out with my hammer spring all the way in and then back off one turn at a time until fps starts to finally fall. Then get to the 20fps below max and good to go.
Didn't get to finish the string as my bolt failed but pretty consistent.
Created: 09-24-2017 06:44:38 PM
Description: .20 Mrod
Notes 1: 3000-2100ish
Notes 2: 1650ish set point
Distance to Chrono (FT): 3.00
Ballistic Coefficient: 1.000
Bullet Weight (gr): 13.730
Altitude (FT): 0.0
Temp: 72 °F
BP: 30.01 inHG
Shots
# FPS FT-LBS PF
47 865 22.81 11.88
46 867 22.92 11.90
45 868 22.97 11.92
44 867 22.92 11.90
43 864 22.76 11.86
42 862 22.66 11.84
41 866 22.87 11.89
40 861 22.60 11.82
39 870 23.08 11.95
38 860 22.55 11.81
37 856 22.34 11.75
36 863 22.71 11.85
35 861 22.60 11.82
34 854 22.24 11.73
33 859 22.50 11.79
32 859 22.50 11.79
31 865 22.81 11.88
30 859 22.50 11.79
29 865 22.81 11.88
28 858 22.45 11.78
27 863 22.71 11.85
26 858 22.45 11.78
25 861 22.60 11.82
24 858 22.45 11.78
23 861 22.60 11.82
22 860 22.55 11.81
21 858 22.45 11.78
20 861 22.60 11.82
19 861 22.60 11.82
18 860 22.55 11.81
17 861 22.60 11.82
16 857 22.39 11.77
15 859 22.50 11.79
14 865 22.81 11.88
13 857 22.39 11.77
12 855 22.29 11.74
11 860 22.55 11.81
10 861 22.60 11.82
9 861 22.60 11.82
8 856 22.34 11.75
7 858 22.45 11.78
6 864 22.76 11.86
5 859 22.50 11.79
4 860 22.55 11.81
3 863 22.71 11.85
2 862 22.66 11.84
1 869 23.03 11.93
Average: 861.21
StdDev: 3.68
Min: 854
Max: 870
Spread: 16
True MV: 861.51
Shots/sec: 0.08
Group Size (IN): 0.00
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My max speed was 940fps with the hammer spring maxed out and it steadily dropped from there as I backed it off so I don't really know my true max speed since I ran out of hammer power lol.
That's a nice shot string by the way! That's what I'm wanting, the shot to shot consistency that you are getting. Jumping 15fps from shot to shot will not help my long range shooting any and I like to try to shoot groups at 75+ yards.
I know it's probably fine as is but it kills me that I think it could be better! I can't just leave stuff be if I think it's off a little just my nature I guess.
Huma makes a reg for the wildcat and it has the handy little scale on it to set pressure. ::) Have you found that scale to be accurate Dillon?
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Jason's explanations are spot on.... If you would like a bit more velocity, and can accept a slightly smaller shot count, then I would just add a bit of hammer spring preload.... That will increase the velocity while on the regulator, and therefore decrease the rise in velocity below the setpoint.... It will quite often also reduce the ES and SD.... PCPs tend to have a greater shot-to-shot variation when operating at pressures above the peak velocity.... That is because the velocity is more dependant on having the exact same hammer strike every shot.... The more consistent the hammer strike, the more consistent the velocity.... Once you are operating near the peak velocity, the gun becomes more tolerant of imperfections in the consistency of the hammer strike.... When the pressure drops below the peak velocity, you tend to just get a lower velocity with each shot, in a very stable and smooth curve....
Tuning on the "knee" of the velocity curve, where you get about a 1% increase in velocity below the setpoint, tends to give you the longest usable shot count, with the smallest ES, while maintaining a reasonable efficiency.... and FPE very near the maximum you can get at that setpoint pressure....
Bob
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I will try that thank you Bob. If the 15.9 grains end up being the best pellet for this gun than I guess I will have to drop the reg pressure. Otherwise running near peak velocity would be too high I'm guessing around 940-950fps? I'll cross that bridge when I come to it I suppose. I learned a lot just from reading these few posts though so it was well worth asking. Thanks all for the help!
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Nothing wrong with 940-950 fps if you gun is accurate there.... I tune for 940-960 as my default, providing I can find a pellet weight that suits the FPE level of the gun.... that is also accurate....
Bob
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That's good to know I always just thought (or assumed) 850-900 was the best bet for accuracy. I don't mind 950 either if the gun likes it.
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From what I gather my pellet options are pretty limited with the smooth twist barrel. Some have had good luck with the 21 grain barracudas I think but mostly just Jsb's. Which is what I'd prefer anyway as long as they work.
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Jason, as usual, can put it more clearly than I can. I just wanted to say one thing, which Jason did touch on. But restricting the transfer port is not the ideal way of adjusting the rifle. It won't change how much air is used per shot, on restricts how quickly it can flow into the barrel. Again, Jason touched on it and mentioned that it was fine for small adjustments and not cutting a rifle's power in half, but I just wanted to reiterate.
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Interesting thread. I'm rather new to PCP's and this is very helpful information.
There is a difference, or maybe I should say a preference, when finding that optimal velocity for accuracy. Bob, you seem to want to be in the 950 range, while Dillon mentioned the 860 range. What determines this? Is it caliber dependent? Pellet weight? Pellet shape?
I have an unregulated rifle (Regal XL in .22), and I am in search of the optimal velocity for accuracy at the distances I'm shooting. I can adjust the hammer spring pre-load, and other than picking various starting fill pressures, that's it for adjusting velocity (of course using pellets of differing weights is also an option).
How do you view maximizing efficiency when dealing with unregulated guns? Is there a way to increase shot count??
Thanks,
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It largely has to do with accuracy, Michael. 860 fps happens to be the sweet spot with JSB 25's in a lot of guns. Bob probably used 950 because it's about the upper limit of diabolo stability. As the pellet enters the transonic zone, it will quickly destabilize. Generally 900-950 fps is the limit, though testing your pellets in your gun is the only way to know exactly where the pellets will destabilize.
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There's no hard and fast rule but in general, the best accuracy at distances >50 yards will be obtained with a domed pellet leaving the muzzle somewhere in the 800-950fps range, with the edge going to heavy pellets in larger calibers. This happens to be a range where the ballistic coefficient is best (again, speaking of domes) but it would be overly simplistic to boil it down to just one characteristic.
The optimal velocity will be dependent on several factors, the most significant of which are probably the pellet and barrel harmonics. Ultimately just have to try different velocities if you are trying to wring out very best from your rifle.
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Thanks Christopher. I'm shooting JSB 15.89's at around 900 fps over a 40-shot string. Accuracy could improve for the distances I'm shooting. The previous owner had it set up for 100 yard bench work, but I'm generally shooting inside 45 yards for pest control and hunting. Not saying accuracy is bad, but I think this rifle can do better. I'll have to lighten the hammer spring pre-load and see what I get over the chrony.
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Thanks Jason. Yes, I've got some testing to do at varying velocities. Better order some more pellets ;D
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Yeah for me 860fps just seems to be the sweet spot where I like my guns to shoot. Good power and velocity but still normally very efficient and quiet. When I tune my rifles this is just the pellets in there respective calibers that I like the 860fps range.
.177 10.34gr
.20 13.73gr
.22 18.13gr
.25 25.39gr
.30 44.75gr
However the AA 16gr I prefer at 900fps. Probably because I don't need to change my tune between the 16 and 18gr .22s ;D
No real rhyme or reason and not saying that's the best speeds but that just seems to be where I've settled.
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Thanks Dillon. I appreciate the information. Sometimes rhyme and reason is just experience over time.
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The least wind drift with round nosed pellets occurs at a muzzle velocity of about 850 fps.... IMO the only reason to shoot much less than that is to increase the efficiency.... ie reduce the amount of air used per FPE of energy produced.... Being a hunter, I am interested in the most energy downrange without picking up too much extra wind drift, so that is why I push towards the 950 fps range.... I get a flatter trajectory and more energy downrange, without a lot of increase in drift.... As long as the pellet I am using is accurate at that velocity, that is my target velocity range.... Some pellets when pushed past about 900 fps tend to spiral, particularly if the twist rate is too high.... I think the MRod (GM barrel, 15.5" twist) with the 34.2 gr. JSBs suffers from this if you push it too fast, and a lot of guys stay just under 900 for that reason... On the other hand, my Hatsan, with a 17.7" twist, has no tendency to spiral with the 34.2 gr. even at 1000 fps, and the way I have the gun tuned, the best part of my string is 950-965 fps, which gives me 70 FPE at the muzzle and over 40 FPE at 100 yards.... with groups at that range between 1.0 - 1.5".... I am willing to put up with less efficiency and a lower shot count to get that kind of long range smackdown.... I get 18 shots within a 4% ES (200 bar fill), and the best 9 shots (1 magazine) are within a 1.6% ES, and that is unregulated, with a 190 bar fill....
Bob
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Don't mean to hijack, but why do pellets have the least wind drift at 850 fps, Bob?
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Don't mean to hijack, but why do pellets have the least wind drift at 850 fps, Bob?
I think it has to do with BC. I remember Brian (EdgunWest) showing in one of his videos that in his .22 matador the Jsb 18.1 had a better BC when slowed down and that it beat the .25 cal in his test for BC. It varies from gun to gun I believe.
The BC test starts at about 12:45....
https://www.youtube.com/watch?v=5MI8e5boDlk (https://www.youtube.com/watch?v=5MI8e5boDlk)
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I think your air volume between the reg and the valve is too small for what the reg is set at. You can either increase the volume by adding spacers between the reg and the valve to increase it's size, or turn down the regulator to maybe around 120 bar to keep your 880ish FPS.
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As you approach Mach 1, the air resistance goes up by at least a factor of three.... This means that the pellet slows very quickly from 1000 fps to 950, a bit less quickly from 950 to 900, less again from 900 to 850, and since the wind drift is proportional to the difference in time of flight in a vacuum and in the real world, the least wind drift occurs below 850 fps.... depending on the pellet the minimum drift may occur as low as 500-600 fps for a wadcutter.... Most good round-nosed pellets, at 50 yards, have the least drift when they start at about 850 fps.... Here is an example....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Wind%20Drift%20JSB%2050_zpsio6vbeef.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Wind%20Drift%20JSB%2050_zpsio6vbeef.jpg.html)
A similar graph for 100 yards would be shifted slightly towards higher velocities, because it is the average velocity that matters.... Notice that at 950 fps the drift hasn't increased much (about 6%), and at 100 yards the percentage increase above optimum would be even less.... which is why I feel OK about using velocities in the mid 900s providing they are accurate....
Bob