20mm re visited

Discussion in 'Piston & Spring' started by Nick G, Sep 29, 2018.

  1. Nick G

    Nick G Active Member

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    Around five years ago , When I was experimenting with reduced diameter comp tube tunes I had a go at a 20 mm, I lashed up a comp tube securing the end plug with dutch dowels (grub screws in the joint) not ideal, and unsurprisingly it blew the end out on the first shot.
    I found it in a draw recently and had another go at it , this time I cross drilled the end plug and used two 4 mm pins , so it shouldn't move, and sealed it with loctite . Its using a 30 mm long bronze skirtless piston with an o ring ,and a very light 2.6 mm wire spring( with 30 mm of pre load) it has a 4 mm long x 3.6 mm trans port , and after a lot of fiddling with lube , and maxing the stroke out ( its in a TX with a full length barrel) its doing 776 fps with 8.4 exacts ( 4.52), that's 11.2 fpe, any thing else i have tried pellet wise makes less power , not tried express yet as i don't have any , If i increase the piston weight or the preload it diesels so its on the limit, The shot cycle however is fantastic.
    Going to shoot it at the range today. I think another 10mm of stroke will put the volume where my 21mm is at and make the power easily, I have an idea that will achieve this in the TX, and if all that works as i want it to I will build a bespoke action making the most of the reduction in diameter meaning a compact rifle , and include some other changes i have found to be useful, ie short port, extra spring room, & solid comp tube lock up in the breech, it will be aluminium , so very lightweight.
    Not really sure if this is a hobby now , or an obsession !
     
    Last edited: Sep 29, 2018
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  2. Sake-San

    Sake-San La Vieille Alliance

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    You are an inspiration Nick but, I begin to suspect you need some medication!
     
  3. hmangphilly

    hmangphilly Not quite a full phil

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    You got a. Bearing on the back of that 30 mm piston head Nick ?

    If not and your using the rear guide to keep it straight , there's 24/25 mm spring room right there
     
  4. Nick G

    Nick G Active Member

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    No bearing just a wasted piston head , I could go down to 22mm long and still have the support, in the Mule TX I intend to push the comp tube up to the breech face , and grub screw the barrel , so the action takes the load and i can set the seal crush with the barrel ,just like a Steyr, that will give another 20mm spring room.In thae new gun I will just make the action longer .
     
  5. hmangphilly

    hmangphilly Not quite a full phil

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    Remember that gun we were going to develop from scratch Nick ?

    got as far as fitting a barrel to a body tube .

    To max out spring room without going unnecessarily long , we partially threaded the barrel, M 15 .plus a bit extra length

    had a plain parallell section of barrel at 13mm .


    partially threaded the action and reamed the rest to 13mm to accept the barrel section and keep us all nice and straight .

    so the barrel is screwed into the action and adjustable for seal crush.

    then locked by a nut , on the outside of the action / body to save room for the comp tube to go futher forward .

    ill find a pic

    we like a pic don't we ?
     
  6. hmangphilly

    hmangphilly Not quite a full phil

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    No need for ugly grub screws squeezing tight spots into your barrel and pushing it out of line.

    sure the nut could be blended to match the big end plug , this was early days and we were going to use the nut as a mount for a shroud and blend the end plug to suit .

    It's highly possible I swiped this idea from someone else ,( that's the kinda fella I am ) but cant remember who .
    Could be my mate Super Dave .
    :)



    IMG_3510.JPG IMG_3511.JPG IMG_3513.JPG
     
    Last edited: Sep 29, 2018
  7. bigtoe

    bigtoe Member

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    Nick, why has it taken you so long to get 20mm working? I have had 3 guns running 20mm on and off for 2 to 3 years, the most efficient is the Diana side lever platform, effortless power. The key all along was the spring with the right spring rate, soft enough to generate power that suits JSB's over the harder pellets, i even showed you suitable springs to use.
    A poly seal would generate power even easier...maybe i will just get some ordered with dovetail fit, O rings can help make things feel to harsh, dual seal gives the best of both worlds.

    Danny's 56 was incredible at 20mm, just pushed to much power with Baracuda Hunter Extremes for some reason, the early ones were soft lead and made power seriously easy...11.2 with exacts and 12.2 with the extremes...nothing at that time would bring the power down, they have changed the alloy now and they do not make power so easy. That comp tube is now in my 52 with a seriously tiny spring and 20mm piston, it produced 5fpe with a stock TX piston at 98mm stroke ;) and makes 11+ with ease in the diana with what i would call medium stroke. I actually think the Diana would make 11 with 105mm stroke 18mm piston although with the harder pellets like superdomes etc.

    here is the spring i use in the 52, its needs scragging...its tiny, efficiency is huge. 2.8mm wire.

    https://photos.app.goo.gl/zeYR7WfoSGrwySmq7

    https://photos.app.goo.gl/HfT7hppcCAMCv7SS7
     
  8. Nick G

    Nick G Active Member

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    It hasn't taken me long to get the 20mm working its been in a draw for 5 years! , recently spent around three hours on it. Not harsh with o rings , never have been, its a Myth.
     
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  9. Nick G

    Nick G Active Member

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    That's Nice, very similar to what i have in mind, mine will have a shroud screwed directly on to the end block, so i could pinch your idea and hide the nut inside it.
     
  10. Nick G

    Nick G Active Member

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    Medication ?Medication? yes definitely need medication!o_O
     
  11. hmangphilly

    hmangphilly Not quite a full phil

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    i dont think the recently featured 21mm lgu single o ring piston could be described as harsh either .
     
  12. Paul_T

    Paul_T Member

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    I've been thinking about this thread on and off since I first read it. I can't get my head around why smaller bore pistons are more efficient.

    Is it just that they are lighter?
    Is it less surface area on the friction surfaces?
    Is it a complicated fluid dynamics thing, where the piston diameter is tuned to the transfer port?

    Sorry if it's obvious but my simple brain can't work it out.:oops:
     
  13. RobF

    RobF Administrator Staff Member

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    Same force over less area equals higher pressure.
     
  14. Nomads HFT

    Nomads HFT Active Member

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    The initial notion behind reducing the bore was to be able to use a lighter piston (to reduce primary recoil) while maintaining piston sectional density, to maintain piston momentum.

    Unfortunately, there is no test data I'm aware of other than chrono readings to give any insight into what's happening inside the rifle during the shot cycle, so any answer to your question can only be speculation.
     
  15. Paul_T

    Paul_T Member

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    Thanks that makes sense.
    So its not necessarily efficiency in terms of energy in verses energy out. Its more of a perceived softening of the shot cycle. (or maybe one naturally leads to the other?)
    If that's the case, does the shot cycle / lock time increase due to the extra swept volume? Or does the reduced mass of the piston balance that out?
    I guess without empirical data it will be a bit unknown, its interesting though.

    Cheers
    Paul
     
  16. Nomads HFT

    Nomads HFT Active Member

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    The time duration of the shot cycle to pellet exit is a function of the barrel length and calibre (obviously), the pellet's start pressure and possibly its kinetic friction, the length of the piston compression stroke, the piston mass, the force acting on the piston at the start of, and throughout, the compression stroke, and probably the transfer port geometry, Paul.

    The diameter of the cylinder only has an effect if other the parameters (listed above) are altered to suit, such as increasing the available piston stroke.

    In the spring/piston airgun, everything depends on everything else.

    If I could get hold of one of these reduced cylinder bore rifles for a few days, it might prove interesting.
     
  17. Adam

    Adam Active Member

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    Or put it another way, lower force required to make the same pressure.

    Softer spring = less input energy. If muzzle velocity (output energy) is the same that means that empirically it is more efficient.
     
  18. Nick G

    Nick G Active Member

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    Made a start on the bespoke 20mm gun today:
    IMG_2282 (1).JPG IMG_2283.JPG IMG_2284.JPG IMG_2285.JPG
     
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  19. hmangphilly

    hmangphilly Not quite a full phil

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  20. Nick G

    Nick G Active Member

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    Shroud is 25mm od , action is 30mm,
    yes back vent, that's why I have gone for the 12mm barrel to max out the volume .
     
    Last edited: Oct 11, 2018 at 4:08 PM

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