Author Topic: what did I do wrong?  (Read 1845 times)

fineredmist_7

  • Newbie
  • *
  • Posts: 12
what did I do wrong?
« on: April 20, 2017, 04:45:57 PM »
I have built a .25 from Buckley's drawing. I first filled it with CO2, and got 580fps. I filled yesterday with 2700 psi of 98/2 argon. (I am a structural welder). The first shot barely made it to the target and 20 yds. I dry fired 10 times, then got 400fps. Dry fired 10 times and got 540fps. Dry fired 10 times and got 680fps. Then the fps hovered around 600 fps for a dozen shots or so. My hammer weights 107g with one inch of travel. I am disappointed in the speeds I am getting, as I had hoped for better performance once I was able to fill with more than CO2. I would appreciate any help finding the problem.


  • Delta, Colorado

Motorhead

  • Sr. Member
  • ****
  • Posts: 395
  • Older & Slower ... Field Target Shooter
Re: what did I do wrong?
« Reply #1 on: April 20, 2017, 05:13:34 PM »
Valve lock  :o .....

What diameter is your poppet valve ?
How heavy is poppet spring ?
How heavy is hammer spring ?
Valves Throat size ?
Hammer travel distance ?

  • Old Hangtown ... California
"Home of Motorheads Air Gun Tuning Service"
    ***** Airguns need love too !! *****

fineredmist_7

  • Newbie
  • *
  • Posts: 12
Re: what did I do wrong?
« Reply #2 on: April 20, 2017, 07:58:50 PM »
I don't know how to measure the weight of the springs. The answers to the rest of the questions should be in the picture.  The hammer travel is one inch.
  • Delta, Colorado

Motorhead

  • Sr. Member
  • ****
  • Posts: 395
  • Older & Slower ... Field Target Shooter
Re: what did I do wrong?
« Reply #3 on: April 20, 2017, 09:35:59 PM »
Over a .265 throat .... using a peek poppet head would go no larger than @ .325"
Transfer passage valve Throat to Barrel IMO ideal @ .230" ... Your way too large in transfer with such a small barrel port.

Ideally crunching area numbers originating at poppet head to throat which establishes entry flow parameters,  given sufficient lift at poppet to expose full area of throat along with all downstream plumbing a gain @ 15% in area for flow losses getting the pulse behind the pellet, you should get good power.
Your excessively large transfer size and small barrel port is absorbing much power potential. The larger poppet head diameter is making the effort to crack the valve open harder then it need be  :P


Bob should give you all the hard numbers ... In my mind the fundamentals of cause & effect are more in play  ???
  • Old Hangtown ... California
"Home of Motorheads Air Gun Tuning Service"
    ***** Airguns need love too !! *****

fineredmist_7

  • Newbie
  • *
  • Posts: 12
Re: what did I do wrong?
« Reply #4 on: April 21, 2017, 06:57:01 AM »
I will take some pics of the valve when I get home tonight. The poppet is delrin.
  • Delta, Colorado

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1513
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: what did I do wrong?
« Reply #5 on: April 21, 2017, 09:03:16 AM »
The basic problem is valve lock.... almost complete at 2700 psi, and then reducing as the pressure drops.... Your velocity is probably peaking somewhere between 1000-1500 psi (just a guess).... Argon (AW 40) is a heavy gas, nearly as heavy as CO2 (MW 44), so at the same pressure you will get very similar performance.... and significantly less than you would get with air (or Nitrogen, MW 28) at the same pressure....

Although your passages between the valve seat and the barrel port are somewhat oversize, that will not drastically affect the power.... Your hammer is heavy enough, and with enough travel to give you plenty of hammer strike with the size valve you have.... Delrin should work fine with a 0.266" throat at 2700 psi, but I would NOT use more than 3000, or you may have problems with it extruding into the throat and have to go to PEEK....

This leaves us with the most likely problem being insufficient hammer spring.... You need to either increase the preload, install a stiffer spring, or both.... I see no other reason why the gun should not perform well with the number you have given.... but you will get more power on air than Argon.... The ideal spring is one that will allow you to reach the maximum velocity at your maximum fill pressure before the spring goes coil bound.... What I mean is, adding more preload does not increase the velocity further at that pressure, you are "flow limited" by the ports.... You then back off the preload until you get a proper "bell-curve" of velocity, like you are getting now, but MUCH flatter.... You are looking for a shot string that starts at, for example, 920 fps, increases to 950, plateaus there, and then falls again.... You stop shooting when the velocity drops back to about 920.... That 30 fps variation in velocity is insignificant out to about 50 yards....

Bob
  • Coalmont, BC

fineredmist_7

  • Newbie
  • *
  • Posts: 12
Re: what did I do wrong?
« Reply #6 on: April 28, 2017, 07:10:18 PM »
The pic shows the old spring, and the current set up. There is a 3/8 OD spring on a 1/4 guide rod, with two one inch long 1/2 springs. I was able to get a 30 shot string before the velocity changed too much.
701.2
699.7
649.3
658.2
655.5
678.4
694.6
686.6
677.2
680.1
666.4
674.2
683.4
683.4
665.4
688.4
689.8
685.9
678.6
684.4
694.7
697.1
692.6
685.8
684.7
674.2
679.5
666.5
664.3
650.3
 this string was from 2500 psi of argon.
  • Delta, Colorado

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1513
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: what did I do wrong?
« Reply #7 on: April 28, 2017, 07:37:47 PM »
For some reason your first couple of shots are high, then you have a reasonable bell-curve from 650-697-650.... with a few high and low values thrown in.... You are definitely getting there.... I would try and investigate the cause of the occaissional shots that don't fit the curve....

Bob
  • Coalmont, BC

oldpro

  • Hero Member
  • *****
  • Posts: 1228
Re: what did I do wrong?
« Reply #8 on: April 28, 2017, 07:40:29 PM »
 Id bet money he just filled his gun and had a couple hot rounds from the warm air, Just a guess.
  • Mount Shasta Ca.

fineredmist_7

  • Newbie
  • *
  • Posts: 12
Re: what did I do wrong?
« Reply #9 on: April 29, 2017, 07:21:12 AM »
The first couple of shots were high because I had the heater on in the truck on the way home from work. I believe that the odd values were caused by inconsistent hammer strikes. I need to do some more inleting to be able to firmly bolt the action to the stock. If the action was able to slide forward at the release of the hammer, that could have taken away from the energy applied to the valve.
  • Delta, Colorado

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1513
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: what did I do wrong?
« Reply #10 on: April 29, 2017, 10:02:07 AM »
If the air pressure was higher for the first 2 shots, the velocity should have been lower.... Perhaps the additional temperature had some effect on increasing the hammer strike?....

Bob
  • Coalmont, BC

fineredmist_7

  • Newbie
  • *
  • Posts: 12
Re: what did I do wrong?
« Reply #11 on: April 29, 2017, 05:40:51 PM »
I used a fairly heavy oil on the hammer. I bet I need to clean that oil off.
  • Delta, Colorado

fineredmist_7

  • Newbie
  • *
  • Posts: 12
Re: what did I do wrong?
« Reply #12 on: April 29, 2017, 08:07:30 PM »
Mr Buckley's drawing says to use a center drill to cut the hole in the poppet for the stem. Which creates a upside down cup shape in the poppet head. I have had three of these collapse. I think the current one is beginning to fail. I know that the tank wasn't empty when I stopped shooting, buy a few hours later it was.
  • Delta, Colorado

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1513
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: what did I do wrong?
« Reply #13 on: April 29, 2017, 08:39:08 PM »
What material did you use for the head of the poppet?.... How much margin did you leave around the edge to create the seal?.... A center drill sounds like more hollow than necessary.... You don't actually need any, a flat poppet will work.... although a slightly concave one will open a bit easier....

Bob
  • Coalmont, BC

fineredmist_7

  • Newbie
  • *
  • Posts: 12
Re: what did I do wrong?
« Reply #14 on: April 30, 2017, 09:39:42 AM »
I used white delrin. There is about one millimeter left to create the seal.
  • Delta, Colorado