My Gk1 after a painstaking tune with 18.1 JSB at 760 fps was producing single hole 5 shot groups consistently, until things went south with a shift of POI downwards and to the left and a group size way over 1.5 “ at 10 yards… happened a few times and what I’ve noticed it was after 20 to 30 rounds and one or two fills out of a 2 liter tank which is making the carbon fiber tank on the Gk1 feel warm. Would give it a rest and try again and things go back to extreme accuracy… puzzling me, can’t figure out why! Could it actually be the barrel warming up and causing a slight warp or the tank transferring heat to the shroud? Anyone had that experience with their Gk1, would appreciate the input.
Huben Huben Gk1 loosing accuracy after 20 to 30 consecutive shots
- Thread starter Sam zakharia
- Start date
When filling air into the gk1 you have to fill it slowly . If done quickly it will generate heat.
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Sam,
With all air pistols, the air tank is limited in volume, not just the GK1. Some are air pressure regulated. These shoot consistently, but this comes at a cost of power and available tank volume simply because the regulator takes up available tank volume, which is already limited in a pistol. The unregulated pistols, like the GK1 are more powerful. So, to manage this POI issue, it is necessary to recharge the air tank after every magazine. As fishing43 said, fill slowly. I actually fill twice to counteract the tank pressure drop from heating and cooling. So, after shooting one magazine, I charge the pistol to 300 bar. I then reload the magazine. By this time the pistol tank has dropped from 300 bar to about 270 bar as the tank cooled while I was reloading the magazine. I then recharged the tank back to 300 bar from the loss of tank pressure due to tank cooling. My GK1s are .25 and have 17 pellet magazines.I set my power to slightly more than 700fps because it provides the best speed consistency over the 17 shot magazine. After one magazine has been shot the tank pressure is about 210 bar.
The next issue you have to contend with is the usable volume of your storage air tank, because only a small percentage of that tank volume is usable before that tank pressure needs to be recharged.. You stated that you use a 2 liter bottle for that purpose. That is way too small for that purpose. I use a 12 liter bottle and my 12 litter bottle gives me about 7 or 8 GK1 charges. I am considering getting a 40 liter steel bottle for that purpose. Of course, it would not be very portable, but it would save the number of recharge sessions required over say a year, as I shoot mostly at home and steel bottles are cheap. I have 3 12 liter bottles. Two are aluminum SCUBA and rated to 220 bar. I also have a 12 liter ACECARE CF bottle, which I use with the GK1s and it is light and handles my away shooting requirements. As a final note on this subject, all these tanks are expensive and the larger tanks require a higher volume compressor with a higher duty cycle rating that is also expensive. The bottom line here is that high pressure air (HPA) is the most expensive part of PCP ownership. Way more than the guns themselves.
With all air pistols, the air tank is limited in volume, not just the GK1. Some are air pressure regulated. These shoot consistently, but this comes at a cost of power and available tank volume simply because the regulator takes up available tank volume, which is already limited in a pistol. The unregulated pistols, like the GK1 are more powerful. So, to manage this POI issue, it is necessary to recharge the air tank after every magazine. As fishing43 said, fill slowly. I actually fill twice to counteract the tank pressure drop from heating and cooling. So, after shooting one magazine, I charge the pistol to 300 bar. I then reload the magazine. By this time the pistol tank has dropped from 300 bar to about 270 bar as the tank cooled while I was reloading the magazine. I then recharged the tank back to 300 bar from the loss of tank pressure due to tank cooling. My GK1s are .25 and have 17 pellet magazines.I set my power to slightly more than 700fps because it provides the best speed consistency over the 17 shot magazine. After one magazine has been shot the tank pressure is about 210 bar.
The next issue you have to contend with is the usable volume of your storage air tank, because only a small percentage of that tank volume is usable before that tank pressure needs to be recharged.. You stated that you use a 2 liter bottle for that purpose. That is way too small for that purpose. I use a 12 liter bottle and my 12 litter bottle gives me about 7 or 8 GK1 charges. I am considering getting a 40 liter steel bottle for that purpose. Of course, it would not be very portable, but it would save the number of recharge sessions required over say a year, as I shoot mostly at home and steel bottles are cheap. I have 3 12 liter bottles. Two are aluminum SCUBA and rated to 220 bar. I also have a 12 liter ACECARE CF bottle, which I use with the GK1s and it is light and handles my away shooting requirements. As a final note on this subject, all these tanks are expensive and the larger tanks require a higher volume compressor with a higher duty cycle rating that is also expensive. The bottom line here is that high pressure air (HPA) is the most expensive part of PCP ownership. Way more than the guns themselves.
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definitely the case but does that actually cause such a drastic shift in poi! Everything else is tight and shooting in the sweet spot between 26mpa and 20mpa but to stay there I’m having to refill after every mag
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Hey Steve thanks for the info, I’m new at this and will eventually build my filling equipment set to the required specs, however the issue with loosing that accuracy and the widened group size after shooting the first round is my concern, mainly for the sake of knowledge and for making sure there is nothing wrong with my set up that might eventually cause a more serious failure. I am shooting in the sweet spot that the gun like with JSB 18 grainers right now and as mentioned earlier between 26 and 20mpa, the question remains could the heat generated by the fill or the barrel heating up after 20 to 30 round cause such a shift? Or do I have something to worry about here! Could the moderator adapter be too tight? I have it at about 6NM just so I can remove the quick disconnect behemoth without loosening the adapter itself. Contacted Huben shop, no answer yet, these people are too slow to respond and not looking forward to dealing with them. Thanks again
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Sam,
You are not giving me much to go on in order to trouble shoot your individual issue. Because of that, I have stated my normal practice. We are all stuck with the gas laws and basic physics. The gas laws state that there is a linear relationship between temperature and pressure. In order for any gas to increase in pressure, it must give up heat and the opposite is true if the gas expands. This is basic adiabatics. Air is not a perfect gas because it is a mix of several gasses, but the gas rules still apply generally.. So when filling the gun, the tank gets warm and when you shoot the gun and tank pressure drops, the tank cools because the gas expands inside the tank and must absorb heat to do that. So now the question is, is what you are seeing normal or not? I cannot say if it is too much or too little, because you do not state your exact starting pressure or your exact ending pressure. I can say that your gun pressure is starting too low for consistent velocity. And you may note that there is in the owners manual, a minimum pressure of around 1.6 mpa. So I suspect you are just above the minimum use envelope of the gun. I would suggest you increase your starting tank pressure and recharge the tank after every magazine as a starter..
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Hey Steve thanks again for the detailed insight. I am starting my session with 260 bars and ending just above 200, the power setting is one turn open, velocity is actually pretty stable, between 760 to 750 fps on my chromy and the deviation is about 3 to 6 fps between shots and when things go wacky on me it’s still holding the same velocities, i‘ve taken off the moderator opened it up inspected it a few times, no debris no pellet rubbing on the edges , the photo is when it’s shooting good, will attach the bad ones when I get home
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I love the way the gun functions and shoots it’s just this chaotic change of performance with no apparent explanation that is triggering me
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Just a follow up to clarify for any misleading info, the gun is shooting great again, the culprit was the quick connect on the behemoth moderator, I had applied son grease for easy removal and at the same time some purple loctite on the adapter thread because I was having the issue of unthreading the adapter when I try to disconnect the moderator. So today I noticed that the behemoth moderator was coming ever so slightly loose and that was causing the widening of the groups and poI shift. Cleaned up the quick connect. Made sure the moderator was tight after every mag and things are looking good again. Thanks everyone and apologies the misleading info.
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Yep learned that the hard way but the huma air mod 30 was a disaster once I went over 20 fpe… will look into a different option in the future maybe a Donny fl one of their bigger models
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