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Semi auto PCP pistol
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Semi auto PCP pistol
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Topic: Semi auto PCP pistol (Read 21514 times - 1 votes)
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JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #100 on:
February 03, 2025, 09:20:08 AM »
Yes, not a bad string. But it's a mix of high and low velocities, something is not repeating consistently. BTW the reservoir capacity is 50cc's
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USA, North Carolina, Saxapahaw
Rob M
GTA Senior Contributor
Posts: 6338
Re: Semi auto PCP pistol
«
Reply #101 on:
February 03, 2025, 09:19:11 PM »
better than a velox with same capacity
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Texas
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #102 on:
February 04, 2025, 12:44:55 PM »
Yep! Here's some progress, 26 shots 671 667 675 669 669 667 662 658 659 654 653 650 647 645 638 634 632 630 628 618 623 624 613 x 606 599. The efficiency is better than this because I have a leak somewhere. A nice declining string, I will try to tune it to a flat string but I don't know how that will go because the blow back design ties everything together. So dialing back the valve duration to flatten the string will also reduce the amount of blowback and will affect the hammers ability to latch back. We will see.
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USA, North Carolina, Saxapahaw
Rob M
GTA Senior Contributor
Posts: 6338
Re: Semi auto PCP pistol
«
Reply #103 on:
February 04, 2025, 08:54:44 PM »
youd have to feed the main tank regged air with a secondary tank making it ugly.. Id keep it as is
«
Last Edit: February 04, 2025, 08:59:36 PM by Rob M
»
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Texas
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #104 on:
February 05, 2025, 06:04:57 AM »
Yeah, it's weird. If it was a standard knock open valve, making the string flat would be easy. But this valve doesn't respond the same way. The first 5 shots look regulated and then it goes back to a descending string. I'll fiddle with it a bit more.
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USA, North Carolina, Saxapahaw
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #105 on:
February 05, 2025, 07:27:13 AM »
Thinking about it, I have kind of a monster hammer spring wound from .043 music wire with an id. of .408 and od. of .5 and 5 coils per inch. I used my (washer/metering lip) on the poppet trick but the gun didn't even notice, the first 5 shots were still the same velocity as without the washer. I might need a bigger washer/metering lip diameter. This hammer spring is like putting a Stormrider hammer spring into a Crosman 2240 or a Jet pistol.
«
Last Edit: February 05, 2025, 07:36:16 AM by JPSAXNC
»
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USA, North Carolina, Saxapahaw
sb327
Expert
Posts: 1863
yes
Real Name: David
Re: Semi auto PCP pistol
«
Reply #106 on:
February 05, 2025, 10:34:28 AM »
The spring obviously plays the most important role in this balance but don’t dismiss hammer weight. Hammer weight will change the dynamics quite a bit in the cycling of the action.
Dave
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USA, OK, Stigler
Dave
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #107 on:
February 05, 2025, 03:40:25 PM »
Hi David, Yes, I'm thinking that the hammer weight could be the problem. I have tried stiffer and lighter valve return springs, stiffer and lighter hammer springs in various combinations. The gun will shoot faster or slower strings, but the strings are always descending. The only thing I can think of is that the hammer weight over comes any combination I try. The valve poppet and seat the stem travel length and all the porting were copied from a Hatsan Jet pistol. The Jet hammer weights about 20 grams the hammer I'm using weights about 60 grams. I'm thinking of lightening the hammer? I have run out of other things to try.
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USA, North Carolina, Saxapahaw
Rob M
GTA Senior Contributor
Posts: 6338
Re: Semi auto PCP pistol
«
Reply #108 on:
February 06, 2025, 09:21:23 AM »
Its always gonna be a decending string , because the mechanisms are intertwined..
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Texas
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #109 on:
February 07, 2025, 07:54:00 AM »
Hi Rob, I made a new hammer that's 30 grams instead of 61 grams it will be finished up today, we'll see if there's any difference. it's worth the experiment anyway.
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USA, North Carolina, Saxapahaw
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #110 on:
February 08, 2025, 07:43:10 AM »
I finished the lighter hammer but have a problem. I bored out the hammer to lighten it and pressed a peek liner to get the id. I needed. But the force of the air blows out the liner. I tried drilling a hole in the side of the hammer to pin the liner in, but the hammer is to hard even for a carbide drill. I'm going to try supergluing the liner in today.
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USA, North Carolina, Saxapahaw
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #111 on:
February 10, 2025, 07:20:18 AM »
The lighter hammer works, but it still shot a declining string. So I put a very large metering ring on the poppet head, essentially doubling the diameter of the poppet. Fired a new string, it was identical to the last string. There is no way you can double the diameter of the poppet and not see a change in the shot string, unless the poppet is not seeing the airflow directly. I think I know what the problem is I'll work on it to day.
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USA, North Carolina, Saxapahaw
subscriber
GTA Senior Contributor
Posts: 7168
yes
Re: Semi auto PCP pistol
«
Reply #112 on:
February 10, 2025, 06:04:57 PM »
James, your tenacity is impressive. I hope you reach your goals. Even if you do not, what you have seems useful already.
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USA
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #113 on:
February 10, 2025, 08:57:28 PM »
Hi subscriber, My firing valve was made in two pieces, the front half is just a support for the valve return spring. To let air in I had drilled 8 3/32" holes around the perimeter of the valve face. But there wasn't enough air coming in to let the poppet respond to the air flow. So I milled out the holes and just left the spring support on two narrow arms, so now enough air is coming in. Still it's shooting a declining string. It's really strange, the design is really no different than the Hatsan Velox and it was a piece of cake to get a flat string. I will experiment again with different springs now that the poppet is responding to the airflow. I have a couple more things to try, then I'll wrap it up and finish raking leaves in the yard.
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USA, North Carolina, Saxapahaw
subscriber
GTA Senior Contributor
Posts: 7168
yes
Re: Semi auto PCP pistol
«
Reply #114 on:
February 10, 2025, 09:00:58 PM »
Perhaps you could add a pressure regulator? Or would that be cheating?
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USA
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #115 on:
February 11, 2025, 04:13:02 PM »
I just realized that I copied all of the geometry from a gun that was designed to be charged to 3600 psi. I think it can take that much pressure. The tube is chromoly .875 od. by .743 id. the valves are 7075 aluminum. Each valve is held in place by 3 hardened 8-32 screws. If you think 3600 psi. would be dangerous please let me know. TIA
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USA, North Carolina, Saxapahaw
subscriber
GTA Senior Contributor
Posts: 7168
yes
Re: Semi auto PCP pistol
«
Reply #116 on:
February 11, 2025, 11:21:00 PM »
Can't answer the danger question without full dimensions of each part, along with material used and heat treatment/state. The air tank contains the most energy at pressure due to its internal volume, so doing some basic calculations for that is highly recommended.
The valve body OD, or tube ID area x the pressure is trying to shear the three screws. This calculation is just as important. Dividing the thrust into 3 for each screw assumes that they bear the load evenly. That depends on how accurately they are placed. Else one or two screws could carry the bulk of the load.
These videos might provide food for thought:
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USA
JPSAXNC
Sharp Shooter
Posts: 742
yes
Real Name: James
Re: Semi auto PCP pistol
«
Reply #117 on:
February 11, 2025, 11:44:50 PM »
Thanks that's helpful. I'm going to stick with a 2600psi fill pressure. I realized all the changes I made to the valve such as valve return springs and metering lips are only affecting the valve duration, I'm thinking changing the valve dwell might flatten the string, I'll try that next.
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USA, North Carolina, Saxapahaw
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Semi auto PCP pistol